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u The only book in which an attempt is made 
to consider in a general manner the development 
of the science . . . may be recommended not 
only to the students of botany but also to the 
general reader."- The Spectator. 

"Professor Harvey-Gibson has undertaken a 
useful work, and he has carried it out well." 

The Times, 


' * It should prove very useful , . . rrmch 
careful research and sound scholarship have 
gone to the making of this book." -Ztoflrw^. 

' * A handy list, arranged alphabetically. It 
gives the derivation, so for as it may be ascer- 
tained with any certainty, and the accentuation 
of most of the names." The Times, 

& C. BLACK, l.m, 4, 5 <&* 6 80HO SQUARE, LONDON, W, t 


New York 




Bombay Calcutta Madras 


From Joly's " Surface History of the Earth," by permission of the 
Clarendon Press. 






C.B.E., D.L., M.A., D.Sc,, F.R.S.E. 





D.Sc., Ph.D., F.I.C. 




Jleto fxitk 





IN editing the second edition of this book opportunity has been 
taken to include a considerable amount of new matter which 
appeared desirable, and to remedy other defects. This has 
involved a very considerable expansion of the modern section, 
which has been largely re-written, and the incorporation of 
chronological and other tables which should add to the value 
of the text. 

From the standpoint of the general reader it is unfortunate 
that men of science, with a few notable exceptions, should 
disdain to popularise their knowledge. Immersed as they are 
in the absorbing intricacies of their own pursuits, inspired by 
a disinterested and single-minded devotion to the elucidation 
of truth, but revelling in a complex symbolism or language 
intelligible only to themselves, it is perhaps hardly surprising 
that they have neither the time nor patience to try to get 
themselves understood by the world in which they live. And 
this world is impatient, with some justice, of the repellent 
terminology and bewildering formulae which confront all 
attempts to understand what the outsider impolitely regards as 

Simple historical treatment, such as is attempted in the 
pages of this book, is probably the best method of approaching 
and appreciating these difficulties; it indicates the laborious 
steps by which, in the past, man groping ever after the un- 
attainable ignis fatuus of absolute truth has reached his present- 
day position, the foundations of which are thus exposed to 
view; nothing better illustrates the fallibility of the human 
mind than the melancholy list of discarded theories of past 
times, and nothing, I venture to think, exercises a more chasten- 
ing influence on man's arrogance in the present. 

September, 1930. 


SOME time ago the author was asked by a friend to recommend 
to him a book of reasonable size which would give him a general 
sketch of the growth of science from early times down to the 
present day, and in which he would also find an explanation, 
written in popular terms, of some of the principal subjects at 
present occupying the minds of scientific men. The author 
found himself unable to cite any such book save the t Short 
History of Natural Science/' by the late Miss A. B. Buckley 
(Mrs. Fisher), published over fifty years ago. That work, 
naturally, did not include any reference to the enormous 
developments in science that have taken place during the 
final decades of the nineteenth century, and in the past 
twenty-five years. There are, of course, numerous books 
dealing with recent progress in the different branches of 
science, but, so far at least as the author is aware, none of 
a popular nature providing a resume of the whole subject in 
a form suitable for general reading. The present volume is 
an attempt to meet that want. 

The author desires to acknowledge his indebtedness to 
several friends who have aided him by their criticism and 
advice on specific points, but more especially to G. G. Chisholm, 
M.A., LL.D., Emeritus Reader on Geography in the Uni- 
versity of Edinburgh; Ralph D. Given, B.Sc., Manager of the 
Industrial Engineering Department of the British Thomson 
Houston Electric Company, Rugby; A. W. Titherley, D.Sc., 
Ph.D., F.LC., Dean of the Faculty of Science and late Lecturer 
in Organic Chemistry in the University of Liverpool; and J. 

Graham Kerr, F.R.S., Regius Professor of Zoology in the Uni- 



versity of Glasgow. He must also express his gratitude to Ms 
friend Professor J. Joly, D.Sc., F.R.S., of Trinity College, 
Dublin, who was so kind as to read through the sections dealing 
with geology. 

April, 1929. 

NOTE. The author, Dr. Harvey-Gibson, died in June, 1929, 
before this book was completed, and while the early proofs were 
in course of revision. As one intimately associated with his 
work during its preparation, the melancholy duty has fallen 
upon me to bring the book into final shape by introducing such 
supplemental matter and amendments as were requisite in the 
interests of clearness and accuracy. But I would crave the 
indulgence of the public to whom this book is presented for any 
defects that may appear. In so vast a subject a concise and 
simple but well-balanced survey should be the aim ; yet the story 
should be intelligible to readers not conversant with the mathe- 
matical and technical intricacies of Science, and must be truth- 
ful : two requirements not easily compatible in a popular exposi- 
tion. This ideal has been my earnest objective in accordance 
with what I know was the spirit and intention of the author. 

September, 1929. 



INTRODUCTION . . - . . i 









i. ASTRONOMY - - - 24 

ii. GEOLOGY - - - - 39 

iii. PHYSICS - - - - 41 

iv. CHEMISTRY- - - - ~ 55 

V. BIOLOGY - - - - 58 




i. ASTRONOMY - - - ~ 73 

ii. GEOLOGY - - - - 89 

iii.* PHYSICS - 105 

SOUND - - - - - III 

LIGHT ----- 114 

HEAT - - - - - 117 



V. BIOLOGY ----- 187 







ill. MODERN GEOLOGY - 342 

iv. PURE CHEMISTRY - - - "377 

Vi. ENVOI ----- 464 



INDEX - - - - - -486 



As we walk along a city street we observe motor-cars, waggons 
and tramways rushing past us but without any obvious mode 
of propulsion; at a seaport we watch great liners, tugboats 
and ferries gliding through the water without sails or oars; 
at a railway station we take our seats in a train and are rapidly 
hauled along an iron roadway to our destination, perhaps 
hundreds of miles away; we enter a dark room and touch a 
button and the room is flooded with light, although we have 
struck no match and lit neither candle nor lamp. The weather 
is inclement and we shrink from facing it to obtain something 
we urgently require, so we lift a receiver from an apparatus on 
the table and give an order to a shop perhaps a couple of miles 
distant. We hand in a message to a post office and within an 
hour or two the message is delivered in New York or Melbourne. 
We sit in an easy chair, with a wireless set, and we may 
listen to a concert being given in Paris or Vienna, or correct 
our watches when we hear " Big Ben " striking noon from the 
clock tower at Westminster. How is all this possible ? Our 
great grandfathers never dreamt of such marvels; our grand- 
fathers heard only whispers of some of them wonders that 
are quite commonplace events to us nowadays. 

We stand at the door on a cloudless night and watch the 
stars twinkling in the heavens, and we can tell how large they 
are, what they are made of, and how far distant they are from 
us. We see the sun rise and set, the moon wax and wane, the 
tides ebb and flow, and weeks, months, even years ahead, we 
are able to say to a minute when the sun will rise or set on a 
certain day, when the crescent moon wiU appear in the sky, 
and at what precise moment it will be high tide at any important 
harbour on the globe. 

On the slabs of shale that often come to the surface from 
the underground workings of a coal-mine we trace the marks of 
leaves, stems and fruits, and of shells and skeletons of animals, 


and we can explain how they came to be there. In a railway 
cutting or on a cliff-face we see rocks arranged in successive 
layers, or twisted and crumpled like sheets of paper, and we 
know how and when they were so laid down and what wrinkled 
and twisted them. 

A table knife, accidentally left overnight on a lawn, rusts 
and becomes heavier; a strip of magnesium wire burns with 
a "brilliant white flame; some flour mixed with saliva turns into 
sugar; a piece of zinc dissolves in dilute acid and gives off 
a gas that explodes in air when a lighted match is put to it; 
we pass an electric current through water and we obtain two 
gases, known as hydrogen and oxygen, which we can cause 
to reunite and form water once more. 

A seed planted in suitable soil grows into a forest tree; roots 
bead, almost invariably, towards the earth and shoots towards 
the light and air; a seagull follows a steamer travelling at the 
rate of twenty miles an hour and keeps up with it with only 
an occasional beat of its wings; a caterpillar changes into a 
chrysalis and a chrysalis into a butterfly; man breeds new 
races of plants and animals for his own pleasure or profit, and 
masters the structure of his own body, so that he may know 
how to keep it in good repair and how to put it in order again 
should anything go amiss. 

If we desire to know about all these wonderful things and 
thousands more, we must study science. 

What is Science ? A great naturalist and philosopher of 
last century Thomas Henry Huxley defined it as " organised 
common sense " or " organised knowledge." The word 
" organised " in the definition is important. To know that 
the tide ebbs and flows twice a day is a useful piece of know- 
ledge, so also is the fact that the moon waxes and wanes and 
travels round the earth, as well as the fact that the earth 
rotates on its axis once in twenty-four hours; but before we 
can see the connection between these items of knowledge and 
their relation to other items, such as the shapes of the continents, 
the direction of the prevailing winds, the temperature of the aii 
and of the ocean, and so on, all this knowledge must be organ- 
ised, co-related or put in order; then only it becomes science. 


Departments of Science. The subject is so enormous that 
for convenience of treatment it must be broken up into depart- 
ments of organised knowledge, which, however, more or less 
interlock. A knowledge of the heavenly bodies, sun, moon, 
planets, stars, comets and nebulae, we call Astronomy; a know- 
ledge of the structure of our own planet, of what it is made and 
how it came to have its present shape, we term Geology; a 
knowledge of the phenomena of heat, light, sound, electricity, 
etc., we speak of as Physics; a knowledge of the inner 
constitution of matter of the universe and the changes it 
undergoes, we call Chemistry; and, lastly, a knowledge of 
living things, animal and vegetable, how they live, grow and 
multiply, is spoken of as Biology, the science of life. 

Even these departments of science are very vast, so that 
portions of each of them are frequently separated off, such as 
Mineralogy, Climatology, Meteorology, Entomology, Anthro- 
pology, and so on. 

All that we now know of science was not discovered in a 
day, or even in two thousand years. Our ancestors, not so very 
long ago, knew nothing or very little of some of these sciences. 
They had never seen an electric lamp, a steamer, or a motor- 
car; they had never despatched a telegram, never spoken 
through a telephone, never even dreamt of broadcasting ! 
What little they did know was often faulty and certainly was 
not organised; but from the very earliest times of which we 
have any record there have been some inquisitive philosophers 
who were not content merely to accept as facts what their 
senses revealed to them, but desired also to know the reason 
for them. Only when they failed to explain the earthquake, 
the thunder aild lightning and other great mysteries of 
Nature, did they ascribe them to the action of supernatural 

The history of science might be likened to the story of 
a winter bud on a tree. In the autumn it lies wrapped up in 
its sheltering scales, the young leaves, axis and growing point 
all planned out in miniature; then follows the winter rest, when 
the bud lies dormant. When spring returns the bud begins 
to open and its leaflets peep out, daring injury and even death 


from untimely frost and enemies of all kinds. At last in early 
summer the shoot emerges with its well-formed axis and wealth 
of foliage, ever progressing towards maturity, yet bearing buds 
in its turn, the promise of even greater developments to follow. 
So also in the history of science there have been four stages of 
growth and evolution. * Long years before our era the first serious 
attempts at scientific enquiry began among the ancient Greeks, 
for the Romans had little or no curiosity about Nature and her 
doings. Science was then in the bud. Then came the winter 
of the Middle Ages, that lasted for over a thousand years, 
though the buds were in large measure preserved by the 
Arabs and the Moors. The long sleep ended, and the Fairy 
Prince that woke the Sleeping Beauty was the grim old monk, 
Roger Bacon. After struggles with stubborn foes, the young 
shoots began to emerge under the fostering care of Kepler, 
Galileo and Newton, and finally the expanded branch appeared 
in all its fulness in the days of Herschel, Faraday and Darwin. 
The buds of the future, however, show no signs of winter sleep ; 
every day almost sees the bourgeoning of new shoots, and the 
tree of knowledge is growing so fast that it is beyond the power 
of any one brain to keep pace with its progress. The most one 
can do is to attempt to picture its growth as a whole, leaving the 
study of its individual branches to those who have leisure and 
inclination to follow their development. 



THALES Solstices and Equinoxes. We must go back 
very far indeed to find the name of the first enquirer into Nature's 
secrets; this was Thales of Asia Minor, who was in his prime 
somewhere about 600 B.C. He was an astronomer of high 
rank, for he was the discoverer of the Solstices and the Equinoxes 
terms we use to this day. The sun appears to rise in the 
east, to sweep across the sky and set in the west, but that 
journey is short in winter and much longer in summer. In 
mid-winter the sun reaches a certain height in the heavens, 
and keeps on reaching the same height for several days in 
succession in other words, it appears to stand still; there is a 
solstice, or " sun-standing." As the sun's height above the 
horizon at noon is not very great, we have a short day and a 
long night; but, as the weeks pass by, the sun rises higher and 
higher and remains above the horizon longer and longer, 
until at length it is twelve hours above the horizon and twelve 
hours below it. This is the time of the spring equinox, when 
day and night are of equal length. Upwards still the sun 
mounts in the sky, the day lengthens and the night shortens, 
until, once more, the sun keeps on reaching the same height 
for several days. This is the period of the summer solstice. 
Another three months pass while the sun gradually fails to 
reach its previous height, and the day and night once more 
become of equal length. This is the autumn equinox. Finally 
the sun sinks to the level it reached at the winter solstice. 
Of course, with a little patience, anyone can follow this cycle 
of solstices and equinoxes for himself, but Thales has the credit 
of having been the first to note and record these phenomena. 

ANAXIMANDER Phases of the Moon. About once a month 
we have a new moon, a slender crescent in the sky, which grad- 
ually thickens until the whole surface reflects the light of the 



sun, and then less and less of it shines on us until it disappears 
altogether. These different conditions are called the " phases 
of the moon/' and are due to its position with regard to the sun 
in its journey round the earth (Figs. I and 2). When the moon 
is between us and the sun its dark face is turned towards us; 
when it is a quarter of its way round its orbit we see half of 
its surface illuminated, and when it is opposite to the sun its 
full face is exposed a full moon. In this last case it might 
be asked how do we come to see a full moon at all ? Will not 
the earth block off the sun's rays from it ? The explanation is 
that the plane of the moon's orbit does not quite coincide with 

f CCQQ(><f f 




S, the sun; E, the earth; M, the moon; N, new moon; QQ, first and 
last quarters ; F, full moon. 

that of the earth round the sun, so that the sun's rays may 
sometimes pass over, and at other times under it. 

All this may seem a very simple matter to us nowadays, 
but to the ancients it meant a knowledge, not only of the fact 
that the moon shines only by reflected light, but also of the 
fact that it revolves round the earth in about a month. These 
facts were discovered by a friend of Thales, called Anaximander, 
who was also the first to teach the Greeks how to measure time 
by means of a sundial. 

PYTHAGORAS Mathematician, Astronomer and Geologist. 
Pythagoras was also a native of the JEgean coast, and lived at 
about the same time. In philosophy, mathematics and natural 
science in all alike he excelled, but it is in his discoveries in 


the last that we are most interested. He taught that the earth 
was a globe which revolved round a central fire; that the 
sea had once been land and the land had been sea; that islands 
had once formed parts of continents; that mountains were for 
ever being washed down by rivers into the ocean ; that volcanoes 
were outlets for subterranean fires; that fossils were the buried 
remains of plants and animals turned into stone. These are 
commonplaces now in every school book on physical geography, 
but were very startling announcements to the Greeks, who saw 
in Etna the chimney of Vulcan's forge, and who believed that 
Olympus was the permanent abode of the immortal gods. 

DEMOORITTJS The Atomic Theory. In these early days 
most philosophers held that the earth was composed of four 
elements earth, water, fire and air; but one thinker, called 
Democritus, a native of Thrace, explained the structure of the 
universe in another way. He said that it was made up of 
infinitely minute particles differing from each other in size, 
weight, shape, etc., and that these particles were indivisible, 
invisible, and indestructible " atoms, J> he called them, i.e., 
particles that could not be cut in two, as the word means. At 
the beginning the atoms were in constant motion, but by and 
by they came together by chance and united in various ways 
to form solids, liquids and gases. Modebti chemistry is founded 
on a somewhat similar idea, which we call the " atomic theory/' 
although we interpret the facts differently. 

HIPPOCRATES The Father of Medicine. These ancient 
thinkers had thus made a beginning in the study of four out 
of the five great sciences. There was left biology, the science 
-of life, and it was only natural that they should commence their 
enquiries in that subject with the living organism that in- 
terested them most the human body. This was done by a 
pupil of Democritus, Hippocrates, who has been called the 
" Father of Medicine/' In those days ailments were thought 
to be inflicted by the gods, and thus the priests were obviously 
the appointed physicians, who made an excellent livelihood 
out of the offerings of the patients who came to seek relief 
at the shrines of ^Esculapius, the god of healing. Hippocrates 
introduced a new system of treatment; he began by making a 


careful study of the patient's body, and having diagnosed the 
complaint, set about curing it by giving dkections to the sufferer 
as to his diet and the routine of his daily life, leaving Nature 
largely to heal herself. This was not, strictly speaking, biology 
as we understand it, but it was at least an attempt at the study 
of one living organism. 

The Academy and the Lyceum PLATO AND ARISTOTLE. 
In Ancient Athens there were two great schools, or rather 
universities, known as the Academy and the Lyceum. The 
head of the former was the renowned philosopher, Plato, and of 
the latter, the equally famous Aristotle. With Plato we have 
really nothing to do 3 for he was not a scientific man indeed, he 
might almost be said to have despised natural science. On the 
other hand, Aristotle, although he also ranked as a philosopher, 
was a keen student of Nature, and it is fortunate that so many 
of his writings on scientific subjects have come down to us, 
although in a roundabout way. The goal he set before himself 
was the preparation of a treatise on the whole of natural science 
as it was then understood, a very ambitious scheme which he 
carried out very unequally. . His writings on what might be 
called the physical sciences were of little value, but those on 
Hying things, and especially on animals, were considered of 
great importance almost down to our own times. He was a 
very competent anatomist, and excelled in his descriptions of 
the forms and habits of animals, but he knew little or nothing 
of the way in which the animal machine worked physiology 
as we term the subject. 

The First Botanist TKEOPHRASTUS. Aristotle also wrote a 
book on plants, which has been lost, although that does not 
matter very much, since his pupil and successor as head of 
the Lyceum, Theophrastus, has left us two treatises on plants 
which, doubtless, contain all that Aristotle had to say on the 
subject. Theophrastus was a native of Lesbos, and, as a boy, 
studied first under Plato and later under Aristotle, whose heir 
he became. Attached to the Lyceum was a botanic garden 
in which he taught his pupils, and where he cultivated the 
plants whose characters and life stories he speaks about in his 
works. He died somewhere about 300 B.C., and with him died 


the science of botany, for we do not hear of a single new dis- 
covery in that subject for over eighteen hundred years. 

The Alexandrine Museum EUCLID AND ARCHIMEDES. A 
new university was meanwhile rising in Egypt to take the place 
of the Lyceum. This was the Museum, or Abode of the Muses, 
founded by Ptolemy Soter, one of the generals of Alexander the 
Great. To this centre of learning Ptolemy and his successors 
on the throne enticed as many of the great philosophers of 
Greece and Ionia as they could induce to settle in Alexandria, 
and begged, bought, and even stole every book they could lay 
their hands on to form the great library, which is said to have 
contained no less than 700,000 volumes. 


One of the great scholars who migrated to the banks of the 
Nile was Euclid, whom every schoolboy knows as the writer 
of a famous book on geometry called " Euclid's Elements," 
but a far greater than he was Archimedes, one of the most 
distinguished men in the history of science. He was born at 
Syracuse in Sicily, 287 B.C., and studied and afterwards taught 
in the Museum. He was a great mathematician, and wrote 
on the circle, the spiral, the parabola, the sphere and the 
cylinder. He was the first to make out the numerical relation 
between the circumference of a circle and its diameter, but his 
best work was done in the science of physics. 

He is often credited with the discovery of the lever, but 
what he really did discover were the laws which governed its 


working. He found out also the principle of a system of 
pulleys, and towed a laden barge single-handed by their aid. 
He invented an endless screw for raising water, an apparatus 
still in use in some countries (Fig. 3) . It consists of a tube wound 
round an inclined axis, one end of the pipe dipping into the 
water to be raised, the other opening over a tank at a higher 
level. The water slowly mounts the tube as the handle is 
turned, as if up a spiral stairway. 

Specific Gravity. One of Archimedes's most important 
discoveries was the law of specific gravity. It is common 
knowledge how he exposed the fraudulent goldsmith who was 
commissioned to manufacture a golden crown for Hiero, ruler 
of Syracuse, by estimating the amount of water displaced by 
equal weights of gold, silver and an alloy of these two metals, 
and so determining the relative amounts of gold and silver in 
the crown. 

The Obliquity of the Ecliptic and the Precession of the 
Equinoxes ARISTARCHUS AND HIPPARCHUS There had been 
several writers on Astronomy since the days of Thales and 
%iaximander; one of these was Aristarchus of Samos, who 
flourished about 250 B.C., another was Hipparchus of Rhodes, 
who lived about a century later. Aristarchus had learnt from 
his predecessors that the earth travelled round the sun, and that 

if one could imagine a line drawn 
from the centre of the sun to the 
centre of the earth, the latter, as it 
performed its annual j ourney , would 
describe a curve on an imaginary 
plane that was called the " plane of 
the ecliptic" (Fig. 4). Now Aris- 

S * R tarchus discovered that the earth's 

FIG. 4. PLANE OF THE ECLIPTIC. . . ,, ,, , ,, 

. axis, joining the north and south 
poles (NP-SP), was inclined to this plane at an angle of 23^. 
This was called the " obliquity of the ecliptic/' Since the earth 
revolves round the sun once a year, the north pole will at one 
time be turned towards the sun and, six months later, away 
from it. When the earth is in the former position it will be 
summer in the northern nemisphere, and when in the latter 



it will be winter, while, of course, exactly the reverse will be 
the case in the southern hemisphere. The intermediate 
positions, three months before midsummer and three months 
before midwinter, will be spring and autumn. In this way 
Aristarchus explained the succession of the seasons. Aris- 
tarchus also was the first to grasp clearly the fact that night 
and day were due to the earth spinning on its axis once every 
twenty-four hours, so that when it was light on one side of the 
globe it was dark on the other. 

Hipparchus's contribution to our knowledge of the heavens 
was what is known as the "precession of the equinoxes/' 
Consider first of all a spinning-top (Fig. 5). On watching it 
carefully it will be noticed that it performs two movements, 
first, a rapid rotation on its own axis, and, second, a much 
slower circular movement (a V} round an imaginary axis (C D) 
perpendicular to the ground through 
the point on which the top is spinning. 
As the speed of rotation of the top de- 
creases this circle in space becomes 
larger and larger until, when the rota- 
tion has nearly ceased, the top tumbles 
over. It is well known that the north 
pole does not point exactly to the Pole 
Star, but to one side of it, and Hippar- 
chus discovered that the precise spot 
towards which the north pole pointed 
was not fixed but changed from year 
to year, describing a small circle in 
space, just as the spinning top does in the preceding illustra- 
tion, and he estimated that this revolution was completed 
in about 26,000 years. Now on looking at Fig. 4 it will be seen 
that this very slow movement of the pole will cause the earth's 
equator (EQ) to cut the plane of the ecliptic at a different point 
each year, and therefore that the equinoxes will never occur 
precisely at the same time in successive years until the circum- 
polar circle is complete. The change is very small indeed, not 
more than fifty seconds of a 'degree in each year, but the dis- 
covery of this forward movement, or " precession/' was one 



of the most striking of the additions to our knowledge of the 
earth's motions made by the ancient astronomers. Wonder 
is often expressed at the remarkable acquaintance with the 
movements of celestial bodies displayed by the ancient Greeks 
and Egyptians; but it must not be forgotten that although 
they had no telescopes and only the crudest of instruments to 
aid them in plotting out the position of stars and planets in 
the sky, they had the immense advantage of having prolonged 
dry summers and clear nights, in marked contrast to what we 
experience in more northern latitudes. 

PTOLEMY. The last of the great men of the Alexandrine- 
school was Ptolemy, or Claudius Ptolemseus, to distinguish' 
him from the reigning house. He was an Egyptian, and lived 
in the middle of the second century of the Christian era. In, 
spite of the fact that both Aristarchus and Hipparchus had 
taught in a quite convincing manner that the earth travelled 
round the sun, and that he was quite familiar with their views, 
seeing that he edited their works, it is extraordinary that 
Ptolemy put forward a theory of the universe of his own which 
was entirely erroneous, but which, nevertheless, was thoroughly 
believed in by the whole world, learned and unlearned, for 
well nigh fifteen hundred years. After weighing pros and 
cons, he decided that the earth was a stationary globe, and that 
the sun, moon and stars revolved round it. This is known as 
the Geocentric Theory, and it held its ground until Copernicus, 
in the middle of the sixteenth century, exposed the fallacies that 
underlay it. Ptolemy also attempted to describe the surface 
features of the earth itself in other words, to write a textbook 
of geography. Here again he had the help of a Greek philo- 
sopher who had lived more than three and a half centuries 
before him Eratosthenes. 

The Measurement of the Earth ERATOSTHENES. Eratos- 
thenes was the royal librarian at Alexandria, and was the firs! 
to draw parallels of latitude by the simple method of joining 
all the places known to him where, on a certain date, the length 
of the day was the same, for he rightly judged that all suet 
places must be at the same distance from the equator. It was 
easy then to draw lines at right angles to these parallels anc 


obtain those of longitude. From this he was led to make an 
attempt at measuring the circumference of the earth, which he 
did in the following manner. Almost on the line of longitude 
that passed through Alexandria there was another city, Syene, 
now called Assouan, and he noticed that, at the summer solstice, 
a pillar there cast no shadow at noon, while another pillar 
of the same height at Alexandria did cast a shadow (Fig. 6). 
Now if a circle be described with a radius equal to the height of 
the pillar at Alexandria (A), the shadow will form a small 
arc on it (C D), and the length of that arc will bear the same 
proportion to the circumference of the circle as the distance 
between Alexandria and Syene (D E) will bear to the circum- 
ference of the earth. Eratosthenes found the arc was -fa * th- 6 
small circle, and that the distance between the two cities was 


5,000 stadia, or, since a stadium is 607 feet, about 574 males. 
Multiplying this figure by 50 gives the circumference of the 
*arth as 28,700 miles, not a very accurate result, but not 
bad for a first attempt. 

STRABO, the Geographer. Strabo, who lived about a century 
before Ptolemy, was a geographer of some note. He made 
a map of the then known world, and, more important still, 
travelled over much of it and described the things he saw. 
Although Vesuvius was not a " burning mountain " in his day, 
he compared its shape with that of Etna, and said it might spring 
into life again, as indeed it did, some sixty years after he died, 
when it destroyed Pompeii and Herculaneum. Strabo had no 
hesitation in saying that the presence of fossil shells in rocks 
many miles from the sea proved that these rocks must have 
been formed from silt brought down by great rivers, like the 
Nile, to form deltas in which these animals once lived. 


The Decadence of Science. After Ptolemy's death, science 
in Alexandria, and indeed in the world generally, began to 
decay. The Romans, who succeeded the Greeks in the dominion 
of the world, had no taste for pure science, and those few scholars 
that were left contented themselves with reading and expounding 
the works of Aristotle and Ptolemy to their pupils, and made 
very few discoveries of their own. 

The bud had passed into its long winter sleep. 



The Fall of Greece and Rome. Ancient history tells us 
how the Roman Empire was overrun by the fierce tribes that 
invaded it from the north and east, and whom we know as 
Goths and Vandals. They were originally inhabitants of the 
almost unknown regions round the Baltic and the basin of the 
Vistula. In the fifth century A.D. these barbarians swept south- 
wards, driven before the Asiatic Huns, until they became next- 
door neighbours of the Romans on the northern and eastern 
fringes of the Empire. How they at length conquered the 
Romans and occupied their territory is another story that 
must be read elsewhere. But an even greater invasion followed 
and completed the destruction of that great race that ruled 
the world from the plains of Mesopotamia to the white cliffs 
of Albion. In Arabia there lived originally a race of shepherds, 
the Arabs or Saracens. These nomads developed warlike 
propensities, burst into Northern Africa and overran the 
whole of it as far as the Straits of Gibraltar, and then passed 
over into Spain and Southern France. They also found 
their way northwards into Syria and Mesopotamia, con- 
quering as they went, until at length they seem to have 
settled down and begun to devote themselves to more peaceful 

Arabian Schools of Learning* Very soon Arabian schools 
of learning began to spring up in a chain round the dying 
Roman and Greek worlds, from Bagdad in the east through 
Cairo on the Nile to Cordova in Spain. The emigrants from 
Constantinople carried with them the writings of the Greek 
philosophers, and these were translated into Syriac and studied 
in the schools of Bagdad. From there they were handed on to 
Northern Africa and so found their way to Spain, where they 
made their appearance in a Latin version. But diligent students 



of the classics as the Arabs and the Moors proved themselves 
to be, there were among them a few who added new fragments 
to scientific knowledge, and about some of these a word or two 
must be said. 

It is difficult to say how much exactly the Arabs learnt 
from the Chaldeans and the Egyptians as well as from the 
Greeks, but one thing is certain they studied chemistry. They 
had read how crude ores might be smelted and metals obtained 
from them. Recognising that gold was the most precious 
metal, many spent their lives and their riches in a vain endeavour 
to convert the baser metals into it, and in the course of these 
experiments they gleaned much information on other matters. 
They found, for instance, that, on heating various substances, 
something invisible was given off which they could collect in 
glass bottles, and to this they gave the name of " spirit," 
likening it to what they believed was the soul. 

GEBER The Father of Chemistry. One of these "al- 
chemists/' as they were called, was Geber, who lived probably 
in the ninth century, and, if the book known as " The Summit 
of Perfection " was written by him, it is the oldest work on 
chemistry with which we are acquainted. It is full of what we 
would call nonsense, but it contains as well much that is novel 
and true. Thus, Geber showed that it was possible to separate 
two liquids by " distillation "e.g., by slowly heating a mixture 
of water and alcohol, the " spirit of wine," or alcohol, came 
off as a vapour at a lower temperature than that at which 
the water began to evaporate. This spirit could be collected 
and, when cooled, condensed separately as alcohol. Geber 
also found that he could obtain "spirits" or vapours from 
solids directly by the process known as " sublimation." One 
of the solids he experimented with was cinnabar, a red mineral 
which is a compound of sulphur and mercury. It is found in 
many parts of the world, but Geber probably obtained it from 
Carniola or from Hungary. On heating this substance a 
vapour was given off which condensed into droplets of quick- 
silver. This metal, which we call mercury, was known to 
Aristotle, and also to a famous Greek physician, named 
Dioscorides, who lived in the first century A.D., who called it 


" hydrargyros," or "liquid-silver," or "quick-silver/' the name 
by which it is still known in some textbooks of chemistry. 

While engaged in these distillations and sublimations, Geber 
noticed that metals, when heated in air, gained in weight, 
but why they should do so he was unable to say. Indeed, it 
was not until nearly the end of the eighteenth century that 
the matter was clearly explained by the great French chemist, 
Lavoisier. Another of Geber' s additions to scientific know- 
ledge was his discovery of the powerful acids, nitric and 
sulphuric; before his day acetic acid, recognised by Pliny in 
vinegar, seems to have been the only acid known. 

Refraction of Light and the Principle of the Lens 
ALHAZEN. While Geber was thus earning for himself the title 
of the " Father of Chemistry," another Arab, Alhazen, was 
born, who in after years made important discoveries in physics, 
and more especially in the subject of light. He studied the 
phenomena of reflection and refraction, drawing attention to 
the bending of the solar rays on striking the earth's atmosphere. 
Since the earth is enclosed 
in a sort of shell of air 
which becomes less and 
less dense the farther away 
it is from the surface, the 
solar rays, he said, must 
be bent inwards and re- 
fracted where they strike 
our atmosphere (Fig. 7, 
B C), so that when we see 
the sun first sinking below 
the horizon, it has really 
set some time before. We now know 
about 8| minutes. 

Alhazen also studied the laws governing the behaviour of 
a biconvex lens when it is fixed in a position to magnify any 
object. Suppose a small object, say a bullet (Fig. 8, B), be 
placed some little distance off, and a biconvex lens (L) be 
'arranged between the eye (E) and it. When the bullet is in 
focus it appears greatly enlarged (B') . The rays of light reflected 



that that interval is 


from the bullet strike the lens, are bent inwards and focussed 
on the retina. But the eye sees all these rays not in bent but in 
straight lines, so that the bullet is seen magnified as at B', It 
was this discovery of Alhazen's that made the manufacture of 
spectacles possible and led in future years to the invention 
of the telescope and the microscope. 

Algebra and Arabic Numerals BEN MUSA. Although we 
are concerned chiefly with discoveries in natural science we 
must not forget entirely the growth of mathematics, and to 
one Arab in particular, Ben Musa, we owe the use of letters 
in place of figures in calculations viz., Algebra, from an 
Arabic word meaning the " joining of parts to make a whole/' 
To the Arabians or Moors in Spain also we probably owe the 


introduction of what we call Arabic numerals i, 2, 3, 4, 
etc. in place of the clumsy Roman ones I, II, III, IV, etc. 
The Arabs, however, did not invent these numerals or Algebra, 
but probably adopted them from India, and there is reason to 
believe that Algebra was known to the Greeks. 

Thus, with the exception of Geber, Alhazen, Ben Musa 
and a few others whose discoveries were not of first-class rank, 
there are no great names on the roll of honour in all these 
centuries that can compare with those of the philosophers of 
ancient Greece and Alexandria, and after A.B. 1000 even 
Arabian science flickered out, and the great Moorish universities, 
such as those of Cordova and Toledo, disappeared altogether. 

But the winter sleep was ending, and the scales that had 
closed over the young leaves in the bud were beginning to open. 
Spring was near at hand. 



The Importance of Experimental Evidence ROGER BACON. 
There are two ways of acquiring knowledge: the first is by 
reading about what other people have done, the other by doing 
things oneself, by examining and cross-questioning Nature. 
She is a past master at hiding her secrets, and she will not 
disclose them save to those who keep on asking her. 

Some who lived in the thirteenth century had begun to 
doubt the truth of many of the statements in the classical 
writings and to look askance at the explanations of natural 
phenomena given by the greatest writers of the past until they 
had tested them by experiment, and of those who lived in the 
years when, to keep up our simile, the bud was beginning to 
open, there was no one who did more to peel off the scales from 
it than a Franciscan monk called Roger Bacon. He must not 
be confused with Francis Bacon, Lord Chancellor of England, 
who lived about 350 years later. It is true, that Lord Bacon 
also insisted on the importance of personal observation and 
experiment in all scientific enquiries, but he contented himself 
with telling others what to do and what not to do, without 
making any discoveries of his own of any importance. 

Roger Bacon was a Somersetshire man, and was born in 
A.D. 1214. After studying at Oxford he went to Paris and 
devoted himself to mastering languages so that he might be 
able to read the works of the Greeks, not only in the original 
text but also in the Syriac and Arabic translations. In 1250 
he returned to Oxford and began experimenting in physics and 
chemistry. But his studies led to his being suspected of 
dabbling in magic and the black arts, and he was soon recalled 
to Paris so that he might be under the immediate eye of the 
heads of the Franciscan Order. The Pope of the period, how- 




ever, Clement IV., encouraged Bacon to write down the results 
of his labours, which he ultimately did. On his return to Oxford 
in 1268 he was again persecuted for his opinions and imprisoned 
for fourteen years, being released only just before he died. 

Bacon has much to say on how to study science, and insists 
on the vital importance of testing by experiment every state- 
ment made by others. " Take nothing on trust " is his motto. 
He wrote on astronomy, and said that the earth was a mere 
speck in the universe and not the central feature in it as most 
people believed. He made an attempt at reforming the 
calendar, and showed how the tides were dependent on the 
position of the moon; but his chief work lay in the science of 
physics, and to what was already known he added much from 
his own observations and experiments. He explained the 
laws of perspective, and gave a quite good account of the 
human eye regarded as an optical instrument, and discussed 
the principles of reflection and refraction of light, and what a 
mirror and a lens do when beams of light fall on them. 
Although he did not actually invent the telescope, he knew 
that, by placing lenses in a certain position between the eye 
and the object, it was possible to " number the smallest particles 

of sand by reason of the 
greatness of the angle under 
which they appear. J ' One of 
his best pieces of work was 
his explanation of the rain- 
bow. He tells us that there 
is nothing out of the way in 
seeing the band of coloured 
light in the sky, for we can 
recognise the same thing in 
water dripping from an oar- 
blade, in the spray from a 
mill wheel, or even in the dew-sprinkled grass in the early 
morning. The colours are not real, he tells us, but are merely 
reflections of different light rays from raindrops, bent or re- 
fracted on entering the drop, reflected from its concave posterior 
surface and refracted again where they pass into the air on the 



way to our eye (Fig, 9). Hence, if a hundred men stood in a 
row with their backs to the sun each would see a different 
rainbow, because each would be looking at different drops of 
water. In the blank spaces above and below the bow in the 
sky the rays, though reflected, never reach the eye at all, 
because the eye is not in the path of these rays. 

It is often stated that Bacon invented gunpowder, but that 
is incorrect; he, in all probability, made it out of charcoal, 
saltpetre and sulphur, but obtained the recipe from some 
Arabian work on alchemy, some say, written by a mythical 
person, called Marcus Graecus. He also experimented with 
air, and found that a lighted candle went out when it was 
covered by a bell-jar, performing, indeed, the very same 
experiment that was made by a certain Oxford doctor, called 
Mayow, 400 years later. All these discoveries seem simple 
enough to us nowadays, but in estimating the real greatness 
of the thirteenth-century monk, we must always bear in mind 
the very credulous age in which he lived and the foolish super- 
stitions and bigoted persecution with which he had to contend. 

The Lodestone and the Compass. For many years after 
Bacon's death science made very little progress; an early frost 
in the springtime had nipped the opening bud and retarded 
its growth. At the beginning of the fourteenth century, how- 
ever, one notable discovery was made that was to prove of in- 
estimable value. It is generally held that it was known to the 
Chinese and also to the Arabs, who doubtless learnt it from 
them through the eastern nations they had conquered, that a 
mineral existed that attracted iron. It was called "lode- 
stone," or "leading stone," and was believed to possess 
magical properties. In one of the tales in the "Arabian 
Nights " an account is given of a ship that came too near 
a hill made of lodestone, with the result that the mineral 
pulled out all the nails from the ship's side, causing it to leak 
so badly that it foundered. In the early days lodestone was 
obtained from Magftesia in Asia Minor, and hence it came to 
be called "magnetic stone," from which we derive the word 
" magnet." It is known to modern chemists as magnetic iron 
oxide. Another remarkable property this mineral possessed, 



and which was also apparently known to the Chinese, was that 
when an iron rod was rubbed with a lodestone and floated on 
a cork in water or suspended in air by a thread, one end of the 
rod invariably pointed north and the other south. Whether 
the Chinese actually did discover the principle of the Mariner's 
Compass we do not know for certain; at least, the sailors of the 
fourteenth century were well acquainted with it, for we read 
of the difficulty in getting a crew to " sign on" in any ship 
whose master, as they thought, steered his vessel with the aid of 
the evil one! Some books record that a person called Flavio 
Gioja was the inventor of the compass, but there are very grave 
doubts as to whether such a person ever existed. 

The Invention of Printing. Halfway through the fifteenth 
century an event took place that, although it cannot, strictly 
speaking, be called a scientific discovery, was destined to 
exert a tremendous influence not only on science but on learn- 
ing in general viz., the invention of printing from movable 
type. In early times pictures and writings were carved on 
smooth slabs of wood, so that the lines and letters were raised 
above the general level of the block. Over these raised lines 
was smeared printer's ink, and the block was then pressed on 
paper and as many impressions as were desired were thus 
obtained. But this was a slow and laborious process, so it 
occurred to one of these engravers, Johann Gutenberg by 
name, that if the letters could be made separately and of 
metal, it might be possible to use the same letters over and 
over again, combining them into words in every conceivable 
way. The great drawback to the old method of transcribing 
by hand was that only very few people could afford to buy 
books, and hence any discovery might take years to reach 
more than a very limited public. The invention of the art 
of printing changed all that, and, owing to the cheaper and 
more rapid multiplication of copies, a far wider public began 
to learn what was going on in the literary and scientific world, 

Not long after the printing of books had become a regulai 
industry in all the chief countries of Europe, another greal 
event, or rather series of events, took place that led to th< 
widening of men's knowledge on an immense scale. 


The Exploration of the Globe. Hitherto the Mediterranean 
had been practically the only sea of any size that had been 
explored. It is true some daring navigators had ventured 
out between the Pillars of Hercules Gibraltar and Mount 
Hacho and crept up the coasts of Spain, Portugal and France 
as far as our own islands, but none had dreamt of seeking 
unknown lands on the other side of the Atlantic. If they did, 
they imagined that they would merely strike the seaboard of 
the countries they already knew something about from their 
land travels in China and the Far East. The first to risk his 
life in this new adventure was the Genoese sailor, Columbus. 
The date of this memorable voyage was 1492, when he sailed 
from Cadiz to find the shores of "Far Cathay/' He never 
found Cathay, but he found an entirely new continent which 
we call America. Fired by his example a Portuguese, Vasco 
da Gama, sailed round Africa and brought home word to the 
astronomers of the new star clusters he had seen in the southern 
skies. But perhaps the most adventurous voyager of all was 
another Portuguese, Magellan, who, early in the sixteenth 
century, started on the first journey round the world. After 
three years his ship came safely back to Seville, and thus 
proved once and for all that the earth was really a globe. 
Alas ! he had not the supreme satisfaction of telling the waiting 
Dons of the wonders he had seen, for he was killed in a fight 
with natives on one of the Pacific Islands, after passing 
through the Straits that now bear his name. 

As we have seen, science as a whole may be divided into 
five sub-sciences viz., astronomy, geology, physics, chemistry 
and biology. We have now reached the stage when the story 
must also be divided into sections. It will not always be 
possible to keep these sciences apart, for the astronomer was 
often a physicist, the physicist often a chemist, the geologist 
both a physicist and a chemist, and the biologist a bit of all 
four. Advance in one science almost always means advance 
in another and sometimes in all; but to keep the story as 
nearly as possible consecutive, we will attempt to follow the 
progress of each science separately within the limits of certain 
periods of time. 



The Heliocentric Theory of the Solar System COPERNICUS. 
Towards the end of the fifteenth century there was born at 
Thorn, on the borders of Poland, a man who was bold enough 
to challenge the truth of the writings of the renowned Ptolemy, 
not to speak of all the high and mighty philosophers, popes and 
bishops who regarded the earth as the centre of the universe. 
This was Nicolaus Copernik, or Copernicus, as he is usually 
styled. After his student days he became a canon of the 
cathedral of Ermland, of which his uncle was bishop. His 
canonry provided him with a salary sufficient for his wants, and 
as the post carried with it only nominal duties, he was able 
to devote himself almost exclusively to his favourite science, 

After a careful study of Ptolemy's theory of the heavens 
he rejected it for that put forward by Pythagoras and Aris- 
tarchus nearly 2,000 years before viz., that, the sun was the 
centre of the universe round which all the planets, including 
the earth, moved. He began his argument by showing that 
what had been so great a stumbling-block to Ptolemy in 
accepting the idea of a moving earth the likelihood of bodies 
on its surface being instantly blown off it if the earth spun 
round on its axis every twenty-four hours was quite imaginary, 
for the atmosphere and all loose objects were carried with the 
earth in its rotation. Again, he argued that if, as Ptolemy 
thought, all the stars were fixed in a gigantic globe, they must 
all be at the same distance from the earth, which was very 
unlikely. Why did they not drop out when the celestial globe 
was revolving at such a stupendous speed ? and if the globe 
were solid, how did a comet manage to get through it ? Of 
course, there were many side problems that Copernicus failed 
to solve, but these were satisfactorily explained by a far greater 
man in the years to follow Sir Isaac Newton. 

Copernicus wrote out his observations and conclusions in 


a famous book " On the Revolutions of the Celestial Orbs/' 
but it was not published until the very day of his death, 
May 24, 1543, almost exactly a century before Newton was 

^ It was scarcely to be expected that the new theory of the 
universe should be at once accepted even by the astronomers, 
for the hold of tradition and authority was too strong. There 
was much spade-work to be done before the Copernican dis- 
placed the Ptolemaic theory, but in the incoming years several 
extremely able men appeared whose labours altered the entire 
outlook on astronomy. The first of these was the son of a 
Danish nobleman, named Tycho Brahe. 

The Rudolphine Tables TYCHO BRAHE. Tycho was 
destined for the legal profession, and, when a mere boy of 
thirteen, was sent to study law at the university of Copenhagen. 
An eclipse of the sun, which took place in 1560, awoke in him 
a keen interest in astronomy, and he spent his pocket-money 
in buying a copy of Copernicus's book and such astronomical 
instruments as he could afford. Instead of burning the mid- 
night oil over treatises on law he extinguished his lamp and 
started to map out the heavens from his study window. He 
proved himself an extremely careful observer and a skilled 
mathematician, and ere long he found that his own star maps 
did not agree with those of his predecessors. Nothing daunted, 
he persevered, and soon convinced himself that his instruments 
were at fault, and that any observations made with their aid 
were likely to be worthless. The obvious thing to do was to 
devise new ones, and the death of his uncle, who had made 
Tycho his heir, gave him the means of doing so. 

After some years spent in Germany he returned to his 
native land, where the King, Frederick IL, became his friend 
and patron, and built for him a magnificent observatory on 
an island in the Sound between Zealand and Sweden. This 
palace Tycho christened " Uraniborg," or "The city of the 
heavens, " and there he laboured for twenty years, until the 
king died. The new king had no taste for learning, and was 
quite unsympathetic with Tycho and his work. The officers 
of State grudged the 400 a year that King Frederick had 


allotted to Tycho as a salary, and generally made things 'so 
unpleasant for him that he was at length compelled to leave 
his " celestial city " and take refuge in Prague, where the 
Emperor Rudolph gave him another observatory, a house and 
a pension. It was here that he carried on the great work he 
had begun at Uraniborg, the making of the calculations and 
the compiling of the famous " Rudolphine Tables " which form 
the basis of the Nautical Almanacks that every sailor uses to 
this day. These tables were unfinished at his death, but they 
were completed by his assistant, an even greater man than 
himself, Johannes Kepler. As a practical astronomer Tycho 
Brahe was supreme, and yet despite his profound knowledge 
of the movements of the heavenly bodies, he never accepted 
the Copernican Theory. Nevertheless the Rudolphine Tables 
formed a magnificent monument to his genius, for they made 
possible the great discoveries of his successors. 

The Laws Governing the Motions of the Planets KEPLER. 
Kepler's youth was a miserable one. His father, though of 
good family, was reduced to keeping a low-class tavern, in 
which Johannes acted as potboy, while his mother was, from 
all accounts, an ill-tempered shrew. He suffered much as a 
lad from the disease that was then the scourge of Europe, 
smallpox, which left him a feeble youngster, quite unfit for 
any heavy manual work. If his body was weak his brain was 
not, and this was recognised by his relations, who sent him 
to the university of Tubingen to study divinity. But he spent 
more time on the study of mathematics and astronomy than 
on the classics, and, when only twenty-three years of age, he 
was elected professor of astronomy at the Styrian university 
of Gratz. 

While he adopted the heliocentric theory, Kepler was a firm 
believer in the old superstition that the planets and stars 
had an influence for good or evil on the lives of men, and on 
this extraordinary delusion was built the ridiculous pseudo- 
science of astrology. Poor Kepler, whose salary was barely 
sufficient to keep body and soul together, had to eke out his 
income by concocting and selling what were called " horo- 
scopes " or charts of the stars at the times of children's births, 




which were supposed to enable the astrologer to predict the 
events of their future lives. 

During all this period he was pondering over the motions 
of the planets round the sun, following Aristotle in believing 
that, since the circle was the one perfect curve, the orbits of 
the planets must be circular. From his mathematical studies,, 
he knew that the Greek geometers, since the time of Pytha- 
goras, considered that there were only five regular solids with 
4, 6, 8, 12 and 20 equal faces respectively, all their edges and 
angles being alike. These solids are the tetrahedron, cube, 
octahedron, dodecahedron 
and icosahedron (Fig. 10). 
Now in Kepler's time only 
five planets were known 
viz., Mercury, Venus, Tetrahedron 
Mars, Jupiter and Saturn 
and Kepler jumped to 
the conclusion that the 
orbits of these five planets 
had something to do with 
these five perfect solids. 
He started with the earth's orbit as a line on a sphere and placed 
round it a dodecahedron whose angles gave him points on an 
outer sphere which was the plane of the orbit of Mars. Then 
he fitted round that sphere a tetrahedron whose angles gave 
points on the plane of the orbit of Jupiter. Outside this he 
placed a cube giving eight points on the plane of the orbit of 
Saturn. Returning to the earth he fitted within the sphere of 
its orbit an icosahedron which enclosed a sphere giving the plane 
of the orbit of Venus, and inside that again an octahedron which 
bounded the orbit of Mercury altogether a very neat and in- 
genious demonstration. " The intense pleasure/' he writes, " I 
have received from this discovery can never be told in words/' 
But, alas ! it was no discovery. We now know that the five 
regular solids have nothing to do with the orbits of the planets. 
Moreover, we know of eight planets nowadays, that they do not 
revolve in circles round the sun, and that their distances from 
each other do not correspond with the distances between 

Dodecahedron Icosahedron 


Kepler's successive spheres as fixed by the angles of the five 
regular solids. 

But Kepler's fanciful explanation of the solar system had one 
important outcome, for it was the means of introducing him 
to Tycho Brahe, whose assistant at Prague he presently became. 
When Tycho died the Emperor appointed Kepler his successor, 
and then followed a peaceful period of eight years, during which 
Kepler completed the Rudolphine tables. With their aid he 
calculated where the planet Mars should be from time to time, 
assuming that it travelled in a perfect circle round the sun. 
But Mars obstinately refused to appear where he was expected, 
and Kepler was reluctantly obliged to confess that if Tycho's 
figures were correct, Mars could not be moving in a circle at all. 
Then the startling suspicion arose in his mind what if the 
mighty Aristotle and all the sages that followed him were 
wrong ? Was it necessary that the planets should revolve in 
circles ? 

Kepler's First and Second Laws of Planetary Motion. 
He first tried the effect of placing the sun not in the centre of 
the circle but in an eccentric position, and then made the 
planet move in its orbit, not uniformly i.e., travelling over 

equal arcs in equal times but at 
varying rates, in such a way that 
it spaced out equal areas in the 
circle in equal times. Thus the 
planet would move more slowly 
when it was farthest from the sun 
and more rapidly when it was 
nearest to it (Fig. n). It would 
thus cover the distance FE or 
B A in equal times but at differ- 
ent speeds, and the areas of the 

FIG< "'" 8 SECONB sectors F s E and B s A would be 

equal. Still Mars would not con- 
form; there was something wrong with the form of the orbit. 
It was then that Kepler boldly discarded the idea of the circle 
as an orbit, for, he said, the theory must be wrong if it does 
not agree with the facts. He next worked through all his 


calculations again, trying ovals of various shapes, and found 
that, though they fitted the facts better, none of them exactly 
met the case. At last he hit on the ellipse, and to his joy 
found that if the sun were in one of the two foci all the condi- 
tions were satisfied and Mars took up his successive positions in 
his orbit quite in accordance with the figures. It is an easy 
matter to construct a diagram that will illustrate Kepler's 
first great discovery. Stick two pins in a sheet of cardboard, 
say, i J inches apart, and loop round them a piece of thread, 
4 inches long, joined at the ends. Stretch the thread with the 
point of a pencil and then, keeping the thread taut, make the 
pencil describe a curve 
round the pins. The 
curve will be an ellipse 
and the two pins will be 
its two foci. Now ima- 
gine the sun to be in 
one of the foci (Fig. 12, 
A) and the planet at B ; 
the line A B is what is 
called a "radius vec- 
tor," which is continu- 
ally changing in length 
as it sweeps round the 
ellipse. When the planet reaches C, the radius vectors A B 
and A C with the arc B C will enclose a sector BAG. Now 
if the whole ellipse be divided into sectors equal in area to 
B A C e.g., DAE and F A G we have the key to the 
problem that Kepler had struggled with for so many years. 
He expressed his discoveries in the first two of his famous 
laws: (i) Planets move round the sun in ellipses, the sun 
being in one of the foci; and (2) a line drawn from the centre 
of the sun to the centre of a planet sweeps over equal areas 
of the ellipse in equal times. 

Kepler's Domestic Troubles The Third Law of Plane- 
tary Motion. In 1612, soon after Kepler had enunciated his 
first two laws, the Emperor Rudolph died, and so he lost a 
powerful patron and friend. His salary was much in arrears, 



and he had a hard fight to make ends meet. Then his wife 
died; three of his children sickened of smallpox, and one of 
them succumbed. His mother got into trouble with the courts 
over a law-suit, and was also accused of sorcery and con- 
demned to the torture. Kepler hurried to her rescue, and, 
though he was successful in saving her from the rack, he could 
not prevent her from being imprisoned. Kepler left Prague 
and took up teaching in the little Austrian town of Linz, where 
he continued his struggle for a living by writing pamphlets 
on astrology. In spite of all these worries and anxieties, and 
without the aid of Tycho's observatory and his tables, he per- 
severed with his astronomical labours, and at last was able to 
announce his third law of planetary motion viz., the square 
of the time of revolution of each planet is proportional to the 
cube of its mean distance from the sun. In consequence of his 
discovery of these three monumental laws, Kepler has been 
styled " the legislator of the heavens," a title he had well earned. 

Worn out in mind and body he fell ill of brain fever and 
died, in 1630, at Ratisbon in Bavaria, at the comparatively 
early age of fifty-nine. 

The First Telescope GALILEO. Though Germany might 
well pride herself on having given birth to so great a mathe- 
matical genius and so skilful an astronomer as Kepler, Italy 
produced perhaps a greater one, Galileo Galilei, who ranks 
with Archimedes as one of the most illustrious scientific men 
the world has ever seen. 

Galileo was born in Pisa in 1564, and his early training 
aimed at fitting him for a medical career, but he had no love 
for such studies, and took every opportunity of reading books 
on mathematics and physics. In the end he had his own way, 
and progressed so rapidly that when he was only twenty-six 
years old he became professor of mathematics in the university 
of his native city. 

During the early years of the seventeenth century the first 
telescope had been invented by a Dutch spectacle maker 
called Lippershey. G^ilep was quick to see the value of such 
an instrument in astronomy, and at once set about making a 
better one for himself. It was, after all, a poor affair, for it was 


made out of a small organ pipe with a lens at either end 
(Fig. 13). The lens (0) received rays (S) from any distant 
object, and brought them to a focus at F, while an eyepiece 
(E P) magnified the image and transmitted it to the eye (E). 

Galileo had studied Copernicus's great work and was 
familiar with Kepler's laws, and so had no doubt in his own 
mind that the sun and not the earth was the centre of the 
solar system. His other discoveries, which had to do with 



physics, had brought him into trouble with the clergy and the 
learned men of Pisa, so he left that city and accepted a pro- 
fessorship in Padua in the free republic of Venice, where he 
remained for eighteen years and where much of his work was 

Having made his telescope, he turned it on to the moon, 
and showed to his astonished friends its mountain ranges, its 
ravines and extinct volcanoes. He found that Venus and 
Mercury went through phases, just as the moon did, and 


demonstrated to his pupils that the Milky Way was composed 
of thousands upon thousands of stars. He gave offence to 
many of his colleagues by finding spots on the sun, and said 
that it revolved on its axis once a month. He discovered also 
that the planet Jupiter had four moons, although some of his 
hearers refused even to look through his telescope lest they 
should see them ! But the telescope alsb showed that, as he 
thought, the planet Saturn, moving in an orbit far outside 


that of Jupiter, was sometimes triple, and at other times single 
with two great knobs, one on either side (Fig. 14). Later on, 
in 1610, he found that Saturn appeared to be single once more. 
How these changes came about was explained half a century 
afterwards by the Dutch astronomer, Huygens, of whom we 
shall speak by and by. 

The Persecution of Galileo. All these discoveries were 
exceedingly distasteful to the followers of Aristotle and to the 
clergy. So long as Galileo remained in a city under the 
Venetian Republic he was safe from persecution, but on an 
evil day he accepted an invitation to become astronomer to 
the Duke of Tuscany, and now being a resident in papal 
territory he became liable to be questioned by the Inquisition. 
He had approved and taught the theory of Copernicus in 
Padua, and he now boldly restated it in Florence; but Florence 
was not Padua. Copernicus'swork, on the " Revolution of the 
Heavenly Bodies/' had been condemned in 1615 and placed 
on the index of forbidden books, and of course Galileo, by 
teaching what it contained, was defying the papal edict. He 
was called to Rome and there ordered not to teach the Coper- 
nican system. As argument with his judges proved of no 
avail; he returned to Florence, where he lived peacefully for 
seven years. 

A new pope, who, as a cardinal, had befriended Galileo, 
came to the throne, and Galileo pleaded with him to lift the 
ban that had been passed seven years before, but the appeal 
was unsuccessful. Then Galileo did a foolish thing: he pub- 
lished "A Dialogue of the Two Systems of the World/ 1 in 
which he made an imaginary supporter of Ptolemy and a 
supporter of Copernicus argue the matter out, with what result 
may easily be imagined. The Ptolemaic philosopher was made 
to look like a fool. Some evil-minded person persuaded the 
pope that " Simplicio/' as the supporter of the Ptolemaic 
system was called, was meant to represent the pope himself. 
Again Galileo was summoned to Rome, and this time was made 
to recant publicly everything he had believed and taught. 
He was then banished to a house near Florence, where he lived, 
virtually a prisoner, for nine years. In addition to constant 


illness he became blind, so that he could no longer explore 
the starry heavens, but gave himself up to the study of physics 
instead, in which subject he made even greater discoveries than 
those he had done in astronomy. He died in January, 1642. 

The Transits of Mercury and Venus GASSENDI AND 
HORROCKS. Between the earth and the sun there are two 
planets, Venus and Mercury, the latter small and nearer the sun, 
the former nearer us. These bodies are also revolving round 
the sun, but much more rapidly than we are, for Mercury 
completes his journey in eighty-eight days and Venus in two 
hundred and twenty-five. Both of them in this way may 
cross the face of the sun from time to time, and these crossings 
are called " transits." But since the orbits of these two planets 
are not quite on the same plane as that of the earth, these transits 
cannot always be observed, for the planets may pass either 
above or below the sun's disc as seen by us. In the case of 
Mercury a visible transit may happen at intervals of seven of 
fifteen years, but in the case of Venus much more irregularly. 
Two transits of Venus succeed each other at an interval of about 
eight years, and then no transit occurs for more than a hundred 
years. A transit of Mercury was first observed by the French 
astronomer, Gassendi, in 1631, and the first transit of Venus 
by a young Liverpool man named Horrocks, in 1639. 

The Size and Distance of the Sun HALLEY. The ob- 
servations of these transits were in themselves very interesting, 
but they were supremely important for another reason; for it 
is by their means that we are able to measure the size of the 
sun and estimate the distance it is away from us. 

At transit Venus is about two and a half times as far 
from the sun as she is from the earth, and the sun's dis- 
tance from us is something like 108 times his diameter; 
what then is the diameter of the sun's disc ? 

Draw two parallel lines, and unite them by a line at right 
angles (Fig. 15, ES). Divide ES into seven equal parts; 
mark a point, V, midway between E and S, arid fix on any 
two points, A and B, equidistant from E. Join A V and con- 
tinue the line to C, and draw B V D in the same way. Of 
course, C D will be equal to A B, and the spots C and D will 




be invisible from A and B respectively, since V comes in the 
way. Shift V to the fifth mark from S, then V will be two and 
a half times as far from S as it is from E, and if the lines A V" G 
and B V 7 F be drawn, the distance F G will be two and a half 
times A B. If V represent Venus, an observer on the earth 
at A will see it as a black spot on the sun's disc at G; another 
observer at B will see it as a black spot at F, and if the distance 
A B be known, F G may easily be cal- 
culated. The moving planet traces a 
line across the sun's disc, but an observer 
at A will see a different track from that 
seen by an observer at B. If the two 
observational points be separated by 
the whole diameter of the earth, ap- 
proximately 7,900 miles, then the dis- 
tance from the track seen by A and that 
seen by B will be 19,750 miles. If the 
distance between the two tracks be T \- 
of the sun's apparent diameter, we 
get 869,000 as the sun's actual diameter 
in miles, and if the mean distance of the 
sun from the earth be 108 times its diameter, that distance will 
be 93,528,000 miles. Of course, these figures are far from 
accurate, and no account has been taken of a number of factors 
which modify the result, but they may serve to show how the 
actual measurements were first made. 

This method is due to the astronomer Halley, who travelled 
to St. Helena in 1676 to map out the constellations of the 
southern skies. In that lonely little island in the South Atlantic 
he observed a transit of Mercury, and it was there that it 
occurred to him that the distance of the sun might be calculated 
from the time taken by a planet to cross the sun's disc. As 
Mercury was too far off he suggested Venus, but, unfortunately, 
there was to be no transit of Venus for eighty-five years, so 
that Halley could not possibly observe it. He succeeded, 
however, in persuading the Royal Society to undertake the 
task when the time came. This they did, and the observer sent 
to the Pacific was the celebrated Captain Cook. The transits 

SUN. (See Text.) 


of 1874 and of 1882 were also carefully observed not only by 
British astronomers but also by those of other nations. In all 
probability there are few now born who will see the nexti 
transit of Venus, which will not take place until 2007. 

The Periodicity of Comets. With Halley's name we 
associate another celestial event. Comets used to be regarded 
as casual wanderers that sometimes paid us visits and then 
disappeared into space, never to return; but Halley studied, 
very carefully, the comet that appeared in 1682, and consulted 
the records of previous visits of a similar celestial vagrant, 
and found that one like it had paid us a call every seventy-six 
years. He boldly predicted that it would return in 1758, 
and back it came, although Halley was not alive to see it. 
Halley's comet reappeared in 1834 and again in 1910, and it 
will almost certainly pay us another visit in 1986. 

SIR ISAAC NEWTON. In the history of science we meet with 
the names of many great men, but all will admit that one of 
these stands out pre-eminent, that of Isaac Newton. He was 
the posthumous son of a small farmer in Lincolnshire, and was 
born in 1642, the year Galileo died. He took no interest in 
crops and cattle, so that after his mother's remarriage his uncle 
took charge of him, and in due course sent him to the university 
of Cambridge, where he studied mathematics. It is recorded 
that he had no patience with Euclid and threw it aside as a 
" trifling book," but he tackled a much more difficult one on 
geometry written by the famous French philosopher, Descartes. 
Even this did not satisfy him, for he discovered what are 
nowadays called the "Binomial Theorem" and the "Differ- 
ential and Integral Calculus." These great mathematical 
works alone would have made Newton's name famous for all 
time, but there was more to follow. 

The Law of Universal Gravitation. In 1665 the Great 
Plague broke out, and the university was closed, so that Newton 
had to return to his home at Woolsthorpe, a few miles from 
what is now the great railway junction of Grantham, and there 
in the quiet country he meditated on the revolutions of the 
planets round the sun. Why did they revolve at all, and what 
kept them in their orbits ? Had gravity anything to do with 


it ? So far as any article near the earth's surface was con- 
cerned, he knew that the force of gravity acting on any falling 
body increased its velocity by 32 feet per second every second. 
Thus a body released at a certain height acquires velocity 
due to the force of gravity, such that at the end of the first 
second the velocity is 16 feet per second. Newton asked him- 
self would the same law apply to bodies in the heavens moving 
at vast distances the moon, for instance ? The moon, he knew, 
was distant from the earth's centre sixty times the earth's 
radius, and if sixty miles made one degree, and since there 
were 360 degrees in a circle, the radius of the earth would be 
about 3,436 miles. Hence the moon was 206,160 miles distant, 
and the force of gravity must be ^^ of what ^ is on the 
earth's surface, because the force of gravitation is inversely 
proportional to the square of the distance from the earth's 
centre 2.0. , taking the distance at the earth's surface as I the 
distance of the moon is about 60 and V=innn5- Newton's 
problem was: Is the force of gravity sufficient to pull the moon 
towards us with an acceleration of jnnnr of tile above 32 feet ? 
If so it is easy to see the earth should pull the moon at the rate 
of 16 feet in a minute, but it did not the figure was only 13-9. 
There must be something wrong with the theory for, after 
repeated trials, he found that his calculations were absolutely 
correct. Although he did not solve the problem till sixteen years 
had passed, and had not mentioned the subject to anyone during 
all that time, we may as well complete the story now. At a 
meeting of the Royal Society in 1682, Newton heard that a 
Frenchman, named Picard, had made a new measurement of 
the earth, and found that a degree worked out to 69-1 miles, 
and not 60, as everyone had hitherto believed. This, of course, 
made the earth considerably larger, and raised the length of the 
radius. Newton at once went over his old calculations in the 
light of Picard's figures, and to his joy the result came out 
exactly right the attraction of the earth was the guiding force 
on the moon. 

Now that the principle was discovered, it followed that not 
only did the earth attract the moon and the sun the earth, but 
every body in the universe attracted every other body inversely 


as the square of the distances separating them. The New- 
tonian law of Universal Gravitation one of the greatest 
scientific discoveries ever mader-is thus enunciated in New- 
ton's own words: " The gravitational force acting between two 
bodies is inversely proportional to the square of their distance, 
and directly proportional to the product of their masses." 

The Tides, Newton's law had far-reaching results. For 
many centuries it had been known that there was an intimate 




connection between the tides and the moon. The high tides 
were highest when the moon was either new or full, and lowest 
when it was in its first or last quarter* The explanation was 
now simple. Both sun and moon attract the mobile ocean, 
and when both attract it at the same 
time in the same direction there will $ 

be high or " spring" tides, but when 
the sun is pulling one way and the 
moon is pulling at right angles to the 
sun, the tides will be " neap " (heljn 
less). In Fig. 16, E represents the 
earth, surrounded by a layer of water 
covering its whole surface. M and S 
represent the moon and sun respec- 
tively. Both earth and moon face 
the sun, and the moon is "new." 
Both sun and moon are pulling on the ocean at the same 
time and in the same direction, causing it to bulge out at 
HH i.e., high tide and lessen its thickness at LL i.e., 
low tide. In Fig. 17 the sun is pulling at right angles to the 
moon, which is now in one of its quarters. Both have a pulling 




effect on the ocean, but the moon being very much nearer has 
the greater power, so that the high tide is not quite so high 
and the low tide not quite so low, since the sun in part neutralises 
the stronger effect of the moon. The matter is, of course, 
nothing like so simple as this when details are considered, for 
in determining the tides exactly we have to take into account 
many other factors, such as the distribution of sea and land, 
the temperature of the water, the prevailing winds, and so on. 
The Cause of the Precession of the Equinoxes. The 
Greek philosopher, Hipparchus, more than TOO years B.C., 
noted that the north pole of the earth was describing a small 
circle in the heavens round the Pole Star, and estimated that 
it took about 26,000 years to complete it. Hipparchus knew 

nothing more than the bare 
fact, but Newton gave the 
reason for it. A model will 
help to make the explanation 
clear. In Fig. 18, A is a fine 
steel ribbon welded into the 
form of a circular band, and 
fastened at what we may call 
the south pole to a small pulley- 
wheel, D. Springing from the 
centre of D is a rod, B, which pierces loosely the ribbon at its 
north pole, so that the north polar region may be pushed down 
the rod, becoming a circle again when the pressure is withdrawn. 
The whole apparatus may be made to revolve by means of the 
larger pulley-wheel C. If the pivot be made to revolve rapidly, 
the north pole of the ribbon will begin to descend, all the more 
so the more rapid the rotation. As a consequence the circular 
ribbon begins to bulge out at its equator and flatten at the 
poles (A') ; it ceases to describe a sphere, but, instead, a spheroid. 
Since the earth spins round on its axis it is not an exact sphere 
but a spheroid, being flattened at both poles. Probably this 
flattening was greater thousands of millions of years ago 
when the earth spun more quickly, but it still retains its 
spheroidal shape. Actually the diameter of the earth at the 
equator is twenty-eight miles longer than that joining the 



poles. The planet Jupiter is much more spheroidal than 
the earth, for his equatorial diameter is approximately 
89,000 miles, while his polar diameter is about 84,500 miles. 
Possibly Jupiter is still in a semiplastic condition, and it is 
rotating much more rapidly than the earth, spinning on its 
axis once in ten hours. Now a sphere attracts as if its whole 
mass were at its centre, but a spheroid does not; in other words, 
the moon does not pull through the exact centre of the earth. 
If a top be spun on its axis, the axis will presently begin to 
describe a cone which will be all the more marked as the speed 
of rotation decreases (Fig. 5). Newton made the calculation for 
the rotating earth, and found it to be just what was required 
to account for the precession of the equinoxes. 

" The Principia." Newton incorporated these and other 
brilliant discoveries in a book called " The Mathematical 
Principles of Natural Philosophy," commonly called "The 
Principia." A genius so great as Newton did not take kindly 
to the preparation of such a book for the press, but he had an 
ardent admirer and friend in the astronomer Halley, who 
undertook the task and carried it to a successful conclusion, 
paying the printer's cost out of his own pocket. 

A few years after the publication of the " Principia/' and 
when he was yet in his prime, Newton seems to have given up 
scientific research and devoted himself to work connected with 
the Mint and the new coinage then being issued. He also 
amused himself with alchemy, hieroglyphics, and biblical 
prophecies, but the stars knew him no more. (His discoveries 
in physics we shall consider later on.) He died in 1727 in his 
eighty-fifth year. 


During the long night of science people took little or no 
interest in what was happening either inside or outside the 
earth's crust, and those who did pay some attention to such 
matters were afraid to write about them because the facts, 
as they saw them, clashed with the teachings of the Church. 


The story of the creation told in Genesis was taken literally, 
which meant that the earth was made in six days, and was not 
more than 6,000 years old. Fossils gave the greatest trouble, and 
all sorts of fanciful explanations were put forward to account 
for them. Some said they were mineral secretions, others that 
that they were "freaks of nature/' but the favourite theory 
was that they were the remains of animals that had been 
drowned in Noah's flood, although how fishes and marine shells 
came to be " drowned " seemed to want some explaining. 

LEONARDO DA VINCI. There was one man, however, who 
studied the subject in the spirit of Roger Bacon, and that was 
Leonardo da Vinci, who lived in the end of the fifteenth century. 
It is very difficult to give in a few words an adequate picture of 
this very remarkable man. Perhaps he is best known as a great 
artist, for every one has heard of his famous pictures, " Mona 
Lisa" and " The Last Supper/' But he was an inventor of 
great ingenuity also. He made water wheels, paddle wheels, 
breech-loading cannon, mining machinery and endless other 
appliances. He worked at magnetism, at steam as a motive 
power, and even studied the circulation of the blood. He held 
that the earth moved round the sun, and thus really forestalled 
Copernicus, and made many discoveries in optics, gravitation, 
friction, heat and light. He was also a distinguished engineer, 
and it was while superintending the cutting of canals in 
Northern Italy that he came on rocks crowded with fossils. 
Using his common sense he made fun of the idea that these 
were either " freaks of nature " or the remains of Noah's flood, 
and said the shells had once lived on the sea, and had been 
buried by silt washed down by rivers from the hills. All this 
was perfectly true, but such views could not overcome the pre- 
judice of people of the fifteenth century, who preferred to 
believe what their fathers had told them rather than the 
evidence of their own senses. 

STENCX There were a few exceptions in this unscientific 
age, and one of these was Nicholas Steno, the Dane, who was a 
contemporary of Newton. He began his career as a physician, 
and settled for a time in Florence, where he wrote his treatise 
OU " Solids enclosed naturally within solids." He saw that 


the layers, or, as we would call them, strata that form the crust 
of the earth, are like pages in a book, telling us of events that 
took place many years ago. He showed how rivers carved 
out valleys, and incidentally created mountains, and gave a 
very fair account of what came to be called the " Denudation 
Theory/' It is true he still believed in the old notion that 
volcanoes are due to enormous subterranean fires, resulting 
from the combustion of substances containing carbon. But all 
through his book we can see how he hesitates to draw the 
obvious inference as to the immense age of the earth, and as 
a Catholic bishop, which he ultimately became, he dared not 
say that the earth was more than 6,000 years old, or that fossils 
were anything else than the remnants of animals that lived at 
the time of the Flood. 

Although they were not strictly speaking geologists, there 
were some men, who lived in the seventeenth century and the 
early years of the eighteenth, who speculated on how the earth 
came into existence. These were men like Descartes, the 
mathematician, the philosopher Leibnitz, and the naturalist 
Buffon, but it would scarcely be worth while to spend time 
over what were largely guesses. Geology had a long way to 
travel before any speculations as to the origin of the earth 
could be made that were based on the facts which the earth 
itself disclosed. The collection of these facts was the task 
of the geologists of the eighteenth century, and of the men 
who gathered them we shall learn in due course. 


The science we have now to study is physics, comprising 
the laws which govern the movements and states of matter, 
the nature of heat, light, sound, electricity, and so on. This 
is an immense subject, and we owe much of our knowledge of 
it to men who were at the same time great astronomers, like 
Galileo and Newton. 

The Foundations of Electricity GILBERT. We have al- 
ready seen that in the beginning of the fourteenth century 


people were acquainted with a mineral called lodestone that 
had very peculiar properties, which were made use of in devising 
the mariner's compass. More than 250 years afterwards, an 
Englishman named Gilbert made some discoveries that were 
the beginnings of a branch of physics that was destined to 
revolutionise the world in days to come electricity. Gilbert 
was a physician, practising in Colchester, who took a 
great interest in this subject. He knew that a piece of 
amber, or fossil resin, after being rubbed with a soft cloth, 
could attract feathers, scraps of paper, or any other light 
objects, but he discovered also that it was not amber only 
that had this power; sealing-wax, jet and many other substances 
acted in the same manner, and he noticed that the power of 
.attraction they possessed was much greater when the air was 
dry and cold than when it was wet and warm. The Greek 
word for amber is ct electron/' and it is from this word that 
we derive the title of this branch of physics, viz., " electricity/' 
The Principle of the Pendulum GALILEO. Great as were 
Galileo's services to astronomy, some hold that his discoveries 
in physics were even greater, and to some 
of these we must now direct our attention. 

There is the oft-told story of his having 
been at a service in the cathedral at Pisa, and 
of his having noticed the swinging of one of the 
lamps hanging from the roof. He counted the 
number of swings, using his pulse as a time 
measurer, and soon saw that the lamp took 
the same time to swing in a small arc as in 
a larger one. He then began to experiment 

ion the subject and discovered the law of 
the pendulum- 
Suspend a disc of metal by a thread so 
that it may swing free (Fig. 19). If the 
thread be about 40 inches long the pendu- 
FIG. 19. PRINCIPLE lum will beat seconds. If it be desired that 
OF THE PENDULUM. it ^y^ b ea t twice in a second, how long 
must the thread be ? One would naturally imagine 20 inches, 
but it is not so the thread must "be only a quarter of the 


length, i.e., ro inches; if three times a second the original 
length, or 4| inches; if four times a second T \ or2j inches, 
and so on. From these figures we get the law of the pen- 
dulum, "the length of the thread is inversely proportional 
to the square of the number of beats in a given time." 
Thus twice the beats, | or the inverse of 2 2 , % or the inverse 
of 3 2 , ^ or the inverse of 4 2 , and it is on this principle that 
our modern pendulum clocks are regulated. 

The Law of Falling Bodies. Galileo's next discovery got 
him into trouble with all the learned people who thought 
that Aristotle was, if not inspired, at least very nearly so. 
Aristotle had said that a heavy weight fell to the ground more 
rapidly than a lighter one. If a penny and a sheet of tissue- 
paper be dropped from the same height at the same moment, 
the penny reaches the ground first, but that is because the 
penny is only slightly buoyed up by the air, since it has a small 
surface as compared with its weight, while the paper exposes 
a large surface to the air, and has a very small weight. If 
both be placed at the top of a long jar from which all the air 
has been extracted by an air pump and both be released at the 
same moment, they will reach the bottom of the jar together. 

Galileo had no air pump, for that instrument was not 
invented till several years afterwards, but he performed a 
notable experiment on a large scale. He took two weights, 
one of 100 pounds and another of I pound, and carried them 
to the top of the famous Leaning Tower of Pisa. This Tower 
is 179 feet high and overhangs its base by i6J feet, so that? 
there is a clear drop from top to bottom. He released the 
two weights at the same moment, and they struck the 
pavement simultaneously. One might imagine that this 
experiment would have convinced those who watched it 
that Aristotle was wrong. Far from it; they had their 
classics, written by the renowned sages of antiquity, which 
had served the learned of all nations for nearly 2,000 years, 
and why should they now throw over these classics because 
an unknown youth, who had begun life as a draper's assistant, 
had dared to contradict writings almost as sacred as the 
Scriptures themselves? So Galileo left the Pisans to their 


dreams and went, as we have seen (p. 31), to become professor 
in the university of Padua, and his subsequent history we have 
already traced. 

After Galileo's banishment to the Villa Arcetri, near 
Florence, he became blind; but, not discouraged by this 
calamity, he spent his declining years in studying the laws 
that govern moving bodies, the discovery of which was quite 
enough to have made him famous for all time. 

The Laws of Motion. If a moving body, he said, is not 
acted on by some force, it will continue to move for ever 
at the same rate and in the same direction in which it is moving. 
If a stone be thrown into the air, and if it be not interfered 
with in any way by any other force, it will fly off into space 
with its initial velocity and at the same angle; but since, instead 
of doing so, it begins to move more and more slowly and 
gradually sinks to the earth, some force or forces must be inter- 
fering with its free movement. First, it encounters the re- 
sistance of the air which reduces its speed, and, second, it is 
drawn down towards the earth by the force of gravity, and 
thus its direction is changed. Once a body is put in motion 
it requires no force to keep it going. So a planet moving 
round the sun continues to do so, but its pathway is always 
changing owing to gravitation exerting a pull on it towards 
the sun's centre. 

Galileo's second law is that when a moving body is acted 
on by any force, the movement alters either in rate or direction 
or both, in proportion to the force exerted and in the direction 
in which the force is applied. If a bowler serves a loose ball 
at a certain speed to a batsman, the latter may hit it to leg 
for four; the batsman has changed the direction and the 
velocity of the ball by applying a new force derived from the 
muscles of his arms transmitted to the bat. Had he not so 
diverted it and at the same time given it an extra jog, and if 
the wicket-keeper did not stop the ball, it would have come 
to rest in the grass, many yards away, when the friction of the 
air and of the grass had overcome its initial velocity. 

The third law is the easiest of all to understand, and is 
familiar to everyone; it is merely this action and reaction 


are equal and opposite; that is so obvious that it needs no ex- 
planation. It was really these three laws and Kepler's laws that 
enabled Newton to build up his theory of universal gravitation. 

The Barometer TORRICELLI. One of Galileo's most dis- 
tinguished pupils was TorricelH, who became professor of 
mathematics at Florence, and who was permitted to visit his 
master in what was really his prison. After Galileo's death, 
in 1642, Torricelli managed to secure some of his papers, among 
them those that dealt with the laws of motion. He smuggled 
them out of Italy and had them printed in Holland, where 
there was no inquisitorial censor. One manu- 
script contained notes on a problem that Galileo 
had failed to solve. He had found that he 
could pump water up to a height of about 
33 feet, but that it obstinately refused to rise 
any higher. It was this problem that Torricelli 
tackled. He argued that if air had any weight 
then it must press equally on the surface of the 
water outside and inside the tubular shaft; but 
if, by the aid of an air pump, the air could be 
removed from the inside of the shaft, the water 
should rise until the column equalled in weight 
the air pressure outside. Torricelli found that 
it took about 34 feet of water to equal this 
atmospheric pressure, which he estimated to be 
about 15 pounds on the square inch. 

Torricelli next used mercury instead of FIG. 20. PRIN- 

water, and, knowing that mercury was fourteen cn>LE OF THE 

t! * v j 4.1. * -4. BAROMETER. 

times as heavy as water, he argued that it 

should rise only T V of the distance that water did viz., be- 
tween 29 and 30 inches. He then took a glass tube about 
3 feet long (Fig. 20), closed it at one end, and filled it with 
mercury and, covering the open end, inverted it in a basin filled 
with the same liquid metal. The mercuryjfell in the tube to B, 
about 30 inches above A, leaving the space, C, quite empty. 
Now it is obvious that if the air becomes heavier than normal 
it will exert greater pressure on the surface at A, and hence 
force the mercury further up into the empty space, C; and 


conversely, if the outside pressure decreases the mercury in 
the tube will fall. It is on this principle that the mercury 
barometer is constructed. It is well to bear in mind that that 
instrument does not foretell the kind of weather to be expected 
in the immediate future; it indicates only the weight of the 
air pressing on the outside mercury. But as the air varies in 
weight according to the type of weather and the amount of 
moisture it contains, we are able to deduce the kind of weather 
we are likely to have. The rise and fall of the mercury may 
be observed directly and measured by an attached scale, or its 
movements may be communicated by transmitters of various 
kinds to the index finger of a dial. 

There is another use to which the barometer may be put. 
We have seen that the atmosphere forms a sort of envelope 
over the earth, which is believed to be anything between 200 
and 500 miles thick. It is naturally densest at the sea level, 
but becomes gradually more rarefied upwards. At a height of 
3^ miles it is only about half as dense as at the surface, and 
at 7 miles only a quarter. Hence the difficulty in breathing 
experienced by climbers of lofty mountains such as Mount 
Everest, which is 5! miles high. It is obvious that if a baro- 
meter be taken up a high mountain the height may be estimated 
by observing how low the mercury sinks in the tube. As a 
mercury barometer is an awkward instrument to carry up a 
hill, another kind, invented long years afterwards and called 
an aneroid, is now used. It depends on the pressure of air 
acting on an empty aluminium drum, but we need not go into 
its construction here, and it did not come into use until the 
middle of the nineteenth century. 

Although Galileo is often said to have invented the thermometer, 
or measurer of temperature, his instrument was not of much 
value. A much better one was devised by a Dutchman called 
Drebbel, who also invented the first submarine. Drebbel 
used spirits of wine in his thermometer, and such instru- 
ments are still employed; but by far the commonest type is 
the mercury thermometer (Fig. 21). It is made by taking a 
glass tube of very fine bore ending in a bulb and filling it with 


mercury for a certain distance up the tube, the end of which 
is at first open. The mercury is then heated, and as it expands 
it rises in the tube, driving all the air out. The tube is then 
hermetically sealed by fusing the open end. As the mercury 
cools it contracts, and, in falling, leaves the upper C F 
part of the tube empty. When the external tem- 
perature rises the mercury expands and once more 
ascends. Early in the eighteenth century the 
thermometer was made of practical use by fixing 
a scale alongside it, divided into steps or grades. 
If we select the freezing-point of water and call 
it zero, and indicate the boiling-point of water 
as 100, we may divide the intervening space into 
100 grades, and so obtain a " centigrade " ther- 
mometer, introduced by the Swedish astronomer, 
Celsius, in 1742. Previously, in 1721, the German 
physicist, Fahrenheit, adopted another scale where 
the lowest temperature that had been noted at 
that time was taken as zero, and the freezing- 
point of water as 32, while 180 above that, 
or 212, was the temperature of boiling water. 
Placing the two scales side by side (Fig. 21) 
the values of the degrees in each may be readily 
compared. There is no doubt that the centigrade 
scale is by far the more convenient, and it is that 
scale that is almost always used in scientific work, 
but the Fahrenheit scale is usually the one found 
on thermometers sold in this country, unless 
the centigrade be specially asked for. 

The Air Pump GUERICKE. Torricelli, in making his baro- 
meter, obtained a space at the top of the tube that contained 
nothing; such a space is called a vacuum. The question then 
arose, Could a vacuum be obtained without using mercury ? 
A method of achieving this was devised by Otto Guericke, 
burgomaster or mayor of Magdeburg in Prussia. The principle 
of his contrivance may be understood from Fig. 22. A is a 
jar in which it is desired to produce the vacuum. B is a 
cylinder firmly attached to A, but opening into it by a close- 



H 190 

80 I8 

































4 8 


fitting valve, D. C is a piston also provided with a valve, E, 

opening upwards. If the piston be pressed down to the bottom 
of the cylinder the valve D closes and E opens 
to allow the air in B to escape. If the piston be 
then pulled up the valve E closes, owing to the 
pressure of the outside air upon it, and the valve 
D opens owing to the efforts of the air in A to 
escape into B. The amount of air originally in 
A will now fill both A and B, and will naturally 
be rarefied. If the piston be again pressed home, 
the rarefied air in B will escape through E and the 
valve D will close. If this performance be re- 
peated again and again the air in A will become 
more and more rarefied, until at last the air pres- 
sure in A becomes insufficient to raise the valve D. 
The machine is by no means perfect, for in addi- 
tion to the fact that there is still some air left in 
A, there is the danger of leakage at the joints and 

Atmospheric Pressure. In order to show his 
discoveries on air pressure Guericke arranged a 
demonstration before the Emperor and his court. 
He obtained two large metal hemispheres whose 
rims were ground to fit each other perfectly 

(Fig. 23). One of these had a nozzle and tap 

which could be attached to an air-pump. When 

the rims were well greased and fitted together, 

he, as far as possible, exhausted the air in the 

sphere and showed that enormous force was 

required to pull the hemispheres apart. It is 

easy to estimate how great this force may be, 

supposing that the sphere contains no air at 

all. If the external surface of the sphere be 

equal to one square yard, or 1,296 square FIG. 23. 

inches, and since the pressure of the atmosphere THE MAGDEBURG 

is 15 pounds on the square inch, it is obvious 

that it would take a force equivalent to more than 8| tons to 

pull the hemispheres apart. 

FIG. 22. 





Attraction and Repulsion in Electrified Bodies. Guericke 
had heard of Gilbert's experiments with amber, sealing-wax 
and other substances, and proceeded to devise new experiments 
of his own. He found that if he suspended a pith ball at the 
end of a silk thread and brought near to it a stick of sealing- 
wax that had been well rubbed, the ball was at once attracted 
to the wax and adhered to it. Having absorbed electricity 
from the wax it was then repelled, and would not again ap- 
proach the wax until it had lost its charge. Guericke rightly 
concluded that an electrified body attracts a non-electrified 
one, but repels one which is similarly electrified. He was also 
the first to notice the spark and the crackling sound when 
electricity jumps across the gap between two electrified bodies 
in short, lightning and thunder on a miniature scale. How 
important all these simple observations were will be under- 
stood when we come to speak of Faraday, who 
worked on the same subject a century and a 
half afterwards. 

Law of Compressibility of Gases BOYLE. 
There was one distinguished man who lived 
about this time who, though he was renowned 
for his work on chemistry, made his mark also 
in physics. This was Robert Boyle. After his 
schooldays were over, he travelled on the Con- 
tinent for five years for the sake of his health, 
for he had always been a sickly youth. He 
returned to England in 1644, and, as he was 
born in 1627, he was only a lad of seventeen 
when he began serious work on science. He 
had heard of Guericke's air-pump, and, in con- 
junction with his friend Robert Hooke, made 
a new and better one, and with its aid began 
to experiment on gases generally. He made 
one very notable discovery with regard to them 
which is always known as " Boyle's law/' He FlG - 
took a bent glass tube (Fig. 24), closed at the 
shorter end, and poured into it some mercury, shaking it 
carefully until it stood at the same level in both legs of the 








tube. The gas in a b was, of course, at normal air pressure. 
He next added mercury to the long leg of the tube up to 
30 inches, c d, representing the pressure of two atmospheres, and 
found that the gas in a b had contracted to a e i.e., to one- 
half its original volume. Again he added 30 inches of mercury, 
and found that the gas was now compressed to one-third. 
Boyle's law of the compressibility of gases is, therefore, that, 
provided the temperature be constant, the volume of a gas 
varies inversely with the pressure to which it is exposed. 

The Nature of Light NEWTON. It was in 1669 that 
Newton became professor of mathematics in the university of 
Cambridge, when he was only twenty-seven years old. His 

duties were to lecture once a 
week on some subject con- 
nected with mathematics, 
physics, or astronomy a 
wide range of choice and quite 
untrammelled by that bug- 
bear of the modern teacher, 
an examination syllabus. 
Newton selected optics, and 
very soon his lectures became 
a description of the dis- 
coveries he himself was mak- 
ing in that subject. From 
his notebooks we have learnt 
that he was busily engaged 
in trying to make better lenses than he could purchase in 
the shops, and yet his own productions did not satisfy him, for 
the images were blurred and indistinct. At last it struck him 
that the fault might lie not in the lenses but in the light itself. 
If a small hole be bored in a window shutter (Fig. 25, W), 
so that light can enter only by that pathway, and if a white 
sheet be hung on the opposite wall, the beam will make a 
circular spot of white light on the sheet. Now obtain a three- 
sided glass prism, P, and introduce it in the path of the ray* 
It will be found that the prism has changed the direction of 
the beam and has spread it out into a narrow ellipse, V R, 



coloured violet at the top and red at the bottom, with indigo, 
blue, green, yellow and orange in between. Although this 
effect of a prism had long been known, Newton was puzzled to 
account for the band of colour being in the form a of very flat 
ellipse. To answer the question place a screen (Fig. 26, S) 
between the prism, P, and the wall and adjust it so as to allow 
only one of the seven colours say, green to pass through a 
hole, H, in the screen. This beam will form a circular green 
disc on the wall, and if a second prism, P, be inserted between 
the screen and the wall, the green ray will be bent out of its 
path, but will not be spread out into any other colours. Since 
the colours merge into each other, it is obvious that the terminal 
ones will have rounded ends, and that the whole will form a 
long narrow ellipse. Thus Newton discovered that white 



light is made up of seven chief coloured rays, which being differ- 
ently bent (refracted) are thus separated by a prism. 

One startling conclusion follows from this experiment. If 
white light be passed through a sheet of red glass, the glass 
does not colour the white light red, as most people suppose; 
it merely blocks off all rays except red. If wMte light falls on 
a grass field we say the grass is green, but that obscures the 
truth. The grass only appears to be so since it absorbs all 
other rays save green and transmits that ray to our eyes, 
creating the sensation in our brains that we call green. If we 
chop up a leaf and soak the fragments in alcohol, we obtain a 
brilliant green solution, and if we hold this solution, con- 
tained in a glass vessel with parallel sides, between our eye 
and the source of white light, we have practically a green 
window-pane that has absorbed all rays save green. 

The belt of colour obtained by splitting up white light by 


means of a prism is termed a " spectrum/' and the instrument, 
known as a " spectroscope/' has proved of very great service 
in analysing the light of various substances, both on our own 
earth and also in the sun and stars. 

Chromatic Aberration. These enquiries were all made 
with the view of finding out why the light that passed through 
a lens made an indistinct image. Since all the coloured rays 
are differently bent, or refracted, when they pass through 
a lens, it follows that they must have different foci, and the 
defects in the lenses that Newton bought or made become at 
once explicable. 

A beam of white light, W W, strikes a lens, L L (Fig. 27). 
[In order to simplify the figure only the outermost rays are 
represented.] As the red rays are least refracted they will come 

w u 


"* L 


to a focus, R, farther away from the focus of the violet rays 
which are refracted most, V, the other rays coming to foci 
between R and V. Now if a white card be placed anywhere 
between V and R, or beyond these points on either side, a 
coloured blur is obtained. This is termed "chromatic aber- 
ration " or " colour wandering/' and Newton decided that, 
since the fault lay in the properties of light and could not be 
remedied, it was useless to continue making lenses. He there- 
fore devoted himself to making telescopes in another way. He 
was, however, too hasty in concluding that chromatic aberra- 
tion was incurable, for, two years after his death, it was dis- 
covered that two kinds of glass, known as " flint " and " crown " 
glass, dispersed light rays differently, and that if these varieties 
of glass were properly combined, one remedied the defects of 
the other. Such lenses are called " achromatic " i.e., not 
giving colour haloes. 



The Reflecting Telescope. If a beam of light strikes a 
concave mirror, the rays are reflected and converge to a focus, 
but there can be no chromatic aberration in this case, for the 
rays do not pass through any glass and are thus not refracted 
before reaching the eyepiece. Newton's telescope (Fig. 28) 
was constructed on this principle. 

A is a concave mirror, made of a mixture of copper and tin, 
placed at the end of a tube. ,, 

A beam of light, B B', strikes g jvr 

the mirror, is reflected from '" 

it, and focussed at C; but, 
before reaching C, it is caught 
by a plane mirror D, set at an 
angle of 45, and reflected to the eyepiece, E, fixed in the side of 
the tube, where the image is viewed by the eye. This is called 
a " reflecting telescope," and it was improved many years 
after Newton's day by another great astronomer, Sir William 
Herschel, and is the type of instrument in use in most of the 
great observatories of the world at the present day, although, 
of course, much more elaborate in detail. 

A Theory of Sound. After having solved these problems 
in optics, Newton turned his attention to the question of sound. 
It had been known since the days of Pythagoras that sound was 
due to the beating of puffs of air on the drum of the ear, but the 


/ 2 / 2 " / 2 "/ 2/2/27 

question that remained unanswered was how the beat was 
carried from its source to the ear. When a pistol is fired the air 
particles in front of the muzzle are forced closely together 
and pass on the bump, so to speak, to those farther away, but 
being elastic they rebound, and hence there is a constant suc- 
cession of bumps and rebounds in the air particles between the 
pistol and the ear. When the air particles are packed closely 
together (Fig. 29, I, i, i), it is called a " condensation," and 


when they are separate from each other, a "rarefaction" 
(2, 2, 2). "The distance between any two condensations (a) or 
any two rarefications (b) is termed a " sound wave." If the 
waves be long and oscillate slowly the "pitch" of the note is 
low; if short and rapidly, it is high. 

That it is really the air that carries the sound may be 
easily proved by placing an electric bell under the exhausted 
receiver of an air-pump and setting the bell ringing. The 
hammer will be seen beating, but no sound will be heard until 
the tap of the receiver is opened and air is allowed to enter, 
when the bell will be heard at once. 

Newton also calculated the speed of sound, and found that 
it travelled at the rate of about 1,100 feet per second, or a mile 
in five seconds. So that if one hears a thunder-clap twenty 
seconds after seeing the flash, one may judge the flash to have 
been about four miles off. 

The Nature of Light the Corpuscular and Undulatory 
Theories. In addition to explaining how light behaves when it 
passes through a prism, Newton also tried to find out what 
light itself is. He thought it was due to infinitely ^ minute 
particles constantly streaming off from luminous bodies, and 
that the impinging of these particles on our eyes gave us the 
sensation we call light. This was known as the " corpuscular 
theory/' but another theory, put forward by the Dutch 
astronomer Huygens, soon became a rival to it. Huygens's 
view was that light came to our eyes in waves just as sound 
came to our ears. But he knew that the air ends a compara- 
tively short distance above the earth's surface, so that he 
had to invent something that would carry the waves from 
the most distant star to our earth, a something thin and 
elastic, filling the whole of space. Newton had previously 
postulated such a hypothetical medium, to which he gave the 
name of " ether " (sether) . It was, Huygens said, the undulations 
of ether that gave us the sensation of light, and hence Huy- 
gens's explanation is known as the " Undulatory Theory." 

The Speed of Light. Newton made no attempt at 
estimating the rate at which light travels. Every one thought 
that light was instantaneous, and they were fully justified 


in so thinking. The only one who appears to have had some 
doubts on the subject was that astute old philosopher, Galileo. 
About 1670, Olaus Roemer, a Danish astronomer, was on 
one occasion studying the revolutions of Jupiter's satellites, 
and noticed that " they did not always pass into Jupiter's 
shadow at the times predicted in the astronomical tables. It 
occurred to him that if light reflected from these satellites 
took a certain time to reach our eyes, these times should differ 
according to whether the satellites were viewed from the earth 
when it was in the part of its orbit nearest to Jupiter (Fig. 30, E) , 
or in the part farthest away from it (E'). He worked out the 

E- E = 1 90,ooo,opo_ m i les 

S -"j ="433,000,000 m lies 
EE', Earth; S, Sun; J, Jupiter. 

difference between the predicted and the actual time of a 
satellite's eclipse, and found it to be sixteen minutes thirty-six 
seconds, or roughly 1,000 seconds. Now the distance across 
the earth's orbit is about 190 millions of miles, and as light 
took 1,000 seconds to travel that distance, its speed was 
obviously 190,000 miles per second, a figure very close to that 
arrived at by the latest workers on the subject. In 1926 an 
estimate was given by the American astronomer and physicist, 
Michelson, which made the velocity to be 187,372 miles per 
second. No wonder most people believe that light is instan- 
taneous, taking as it does.only about eight and a half minutes 
to travel from the sun to our earth 1 This enormous velocity 
of light is a number that ought to be remembered, for we shaU 
meet with it again when we consider the advances m science 
that have taken place in our own time. 


We need not spend much time over what passed for 
chemistry in the seventeenth century. It is only some 300 
years since people believed that there were only four elements 


as Aristotle had taught viz., earth, air, water and fire. Now, 
of course, earth is recognised as a very complex mixture of 
compounds of all sorts of elements; water is a compound of 
two elements; air is mainly a mixture of two elements, and 
fire is not a substance at all. The alchemists held there were 
only three elements; sulphur, mercury and salt. 

In spite of such crude ideas, a good deal of research of a 
kind had been carried out; for the Greeks left behind them 
recipes of how to extract metals from their ores and how to 
make alloys of two or more metals. Geber discovered some 
of the strong acids, and showed how to distil liquids and to 
sublimate solids; but a science of chemistry must be an 
organised body of facts, not merely a collection of pieces of 
information on unrelated subjects, however interesting and 
important in themselves. 

The Definition of an Element. Robert Boyle, the dis- 
coverer of the law of the compressibility of gases, has often 
been called the " Father of chemistry," and his chief claim to 
that title rests on the book called " The Skeptical Chymist," 
in which he ridiculed the old ideas about elements. He defined 
an element as something that cannot be broken down into two 
things different from itself; it is impossible to split gold into 
anything but gold, or silver into anything but silver, and so on. 
But, although his definition of an element is correct, his facts 
were often wrong, for he thought that lime, potash and 
magnesia were elements because he could not decompose them. 

The Phlogiston Theory. A curious theory of combustion 
was started about the end of the seventeenth century by two 
German chemists, Becher and Stahl. It was based on an 
entirely erroneous idea, but it was firmly believed in at the time, 
and retarded the advance of chemistry for nearly a century. 
This was called the " phlogiston theory," from a Greek word 
meaning " something inflammable"." It was held that every 
substance that was combustible was composed of a " calx," or 
ash, and a certain amount of a substance called " phlogiston/ 1 
Some bodies, it was thought, had much phlogiston and very 
little calx in their composition, and such bodies e.g., charcoal 
burnt readily, while others, such as lime, had an excess of 


calx and very little phlogiston. When a combustible body 
was burnt it lost its phlogiston to the air and could get it back 
again only from the air, or from some other body that had a 
surplus of it. 

The Nature of Air. Boyle had made some interesting 
experiments, partly chemical, partly biological, which led 
to some very important results later on. He showed that a 
lighted candle went out when it was placed under a bell- 
jar from which the air had been extracted, and that small 
animals died when similarly treated (cf, p. 21). Hence he 
concluded that air was essential to combustion and to life. 
He gave an account of his experiments at Oxford, where one of 
his audience was a young doctor, called Mayow. Mayow 
was born in Cornwall in 1645, an d, a f ter taking his degree, 
began to practise at Bath. He died when he was only thirty- 
three, so that he must have been a mere youth when he heard 
Boyle lecture. 

From what he learned he concluded that there must be 
something in the air that supported flame and life, and that 
this something could be only a relatively 
small part of the air, since a candle went 
out long before all the air in the bell-jar 
was exhausted. 

Obtain a large bell-jar (Fig. 31, B) 
with a tap at the top, and divide its con- 
tents into six equal parts, and also a 
sufficiently large basin, A. Fix a lighted 
taper in a lump of plasticine at the bottom 
of the basin, T, containing water, and 
quickly cover it with the jar until it stands 
opposite the zero mark, the tap mean- 
while remaining open; then close the 
tap. At first the water will sink below 
zero, because the taper has heated the 
air and caused it to expand, thus forcing the water down. 
Presently it will rise, and go on rising so long as the taper 
continues to burn. When the taper goes out, and when the 
bell-jar and its contents have stood, it will be found that the 



water will have risen to a higher level nearer the dotted line L, 
showing that something of the air originally in the jar has 

Exactly the same kind of thing happens if a mouse, or 
other small animal, be placed in the jar in lieu of the taper; 
the water rises and the animal dies. Mayow, from his ex- 
periments, decided that there were two gases in the air, one of 
which, amounting to about one-fifth of the whole, supported 
combustion and life; the other did neither, but was neutral, 
or inert. The first he called " spiritus nitro-sereus/' or " fire 
air," but to the other he gave no name. We know the 
first nowadays as oxygen and the latter as nitrogen. It is 
often stated in textbooks of chemistry that a chemist, called 
Priestley, discovered oxygen, exactly a century later, but the 
honour might be given to Mayow, although Priestley redis- 
covered it ajid was the first to isolate it. 

Chemistry was thus in a very backward state in the sixteenth 
and seventeenth centuries, and for two reasons first, because 
the facts that had been discovered were relatively few in 
number; and, second, because what was known was not 
organised, and it is only organised knowledge that constitutes 
a science. 


The sixteenth and seventeenth centuries were rich in great 
names connected with the science of biology the science of 
life. There were botanists who recommenced the study of 
plants where Theophrastus left off, 2,000 years before; there 
were zoologists who carried on the story of the animal world 
as begun by Aristotle, and there were men who eagerly explored 
the structure of the human body and proved themselves worthy 
successors of Hippocrates. 

Nowadays, when we speak of a man as a botanist or a 
zoologist, we mean one who endeavours to study a plant or 
an animal from every point of view, as a structure made up of 
definite parts, as a living machine, feeding itself, adapting 


itself to its surroundings and multiplying itself, or producing 
progeny like itself. The biologist also studies how plants are 
related to other plants, and animals to other animals, where 
they are to be found, and how they came into existence. 
The biologists of the sixteenth century attempted something 
in the way of describing the structure of the organisms they 
studied, and tried to classify them in groups, but they knew 
little or nothing of how they lived or how the machines worked. 

The Herbalists. The first botanists were what we call 
" herbalists " i.e., men who described the structure of plants 
growing round them, chiefly that they might be the more 
easily recognised as sources of food or drugs. Among these 
the more notable were Brunfels, Fuchs, Bock and Cordus, ail 
of whom lived before the close of the sixteenth century. The 
chief among the earlier zoologists was Gesner, who wrote an 
elaborate work on natural history, giving an account of all the 
animals then known. 

The Pioneers in Modern Medicine. In medicine one great 
name had come down through the centuries, that of Galen, 
a physician of Asia Minor, where he was born in the early part 
of the second century, and who, after studying at Alexandria, 
became the medical attendant on the Emperor Commodus at 
Rome. His books were regarded as the last word on every 
thing concerned with health and disease, and to question them 
was heretical in the highest degree. But there was one man 
who lived in the beginning of the sixteenth century who dared 
to criticise Galen, a learned chemist, as chemistry was under- 
stood in those days, a man who set himself the task of exposing 
all the humbug and fraud that passed for medicine, and who 
fought for common sense in the treatment of disease. This 
man was Paracelsus. Like all other great reformers he got 
himself into trouble, not this time with the Church, as Galileo 
did, but with the universities and the medical profession. He 
fought against authority as represented by Galen, and preached 
the sound doctrine that Nature herself was the great healer, 
and that the physician's business was to provide her with what! 
she required to fit her for the battle she was waging against 


The First Anatomist VESALIUS. But Galen's teaching was 
attacked by others, who vigorously criticised his ideas about 
the structure of the human body. The chief of these was 
Vesalius, professor in Padua, where he taught anatomy by 
dissecting the body before his students, and insisting that if 
the actual dissection contradicted what Galen had said, then 
Galen must be ignored. 

The Circulation of the Blood HARVEY. The fame of Padua 
as a school of anatomy and medicine tempted to it students 
from all over the world, even from far-away England, and one of 
these was William Harvey, whose name will be famous for all 
time as the discoverer of the circulation of the blood. He was 
the son of a Kentish farmer, and was born in 1578. About 
twenty years later, after studying at Cambridge, he went to 
Padua, where he worked under Vesalius's successor, Fabricius, 
who discovered the valves in veins. When Harvey returned 
to England he took his doctor's degree at Cambridge and 
settled down to practise medicine in London, but at the same 
time gave lectures on the structure of the body and how the 
various organs worked together for the good of the whole. It 
was during this time that he prepared his great work, " On the 
Movements of the Heart and Blood/' a book quite as famous 
in its way as Copernicus's " Revolutions of the Celestial 
Bodies." In order to appreciate fully Harvey's epoch-making 
discoveries, it is essential to know what people believed on the 
subject before his time. 

Aristotle thought that blood was made from food in the 
liver, that it flowed from the liver to the heart, and went from 
there to all parts of the body by way of the veins. The arteries 
were the means by which a " spirit/' or " very subtle essence/' 
was distributed through the body, although Galen thought they 
might contain blood also. This was doubtless .because the 
arteries in a dead body were found to be empty. The pumping 
that caused the movements of the blood was believed to be the 
act of breathing. The heart was divided into two chambers, 
right and left, which communicated with each other through 
a porous partition, and there were two kinds of blood, one 
that flowed from the liver to the right side of the heart, 


FIG. 32. 




from which it went to the lungs and other organs by the>veins, and 
another kind which followed the same course by the arteries. 
It was known that the heart expanded and contracted, but it 
was thought that the expansion was due to a " spiritus " or gas 
inside it. The whole question was thus in a niost confused 
state, and the so-called explanations were entirely misleading. 

Now we come to Harvey's discoveries. To begin with, he 
recognised that there were two kinds of bloodvessels, arteries 
and veins, and that, in the living body, they bcfth 
contained blood. He found that, if he tied - an 
artery, it began to swell and throb on the side next 
the heart, and hence it was obvious that the blood 
flowed from the heart to the limb whose artery he 
had bandaged. When he tied a vein, say, in* the 
arm, the part beyond the ligature i.e., on the side 
farthest from the heart began to swell, but there 
was no throbbing. That meant that the blood was 
flowing from the limb to the heart by the vein, but 
was not actually being rhythmically pumped'there. The pur- 
pose of the valves that Fabricius had found in the veins was 
now obvious ; they did not prevent blood 
flowing to the heart (in the direction of 
the arrow in Fig. 32^ A), but they did 
prevent it from flowing back from the 
heart (B) ; and, further, each throb or 
beat in the artery f ollpwed a contraction 
of the heart, not, as was commonly sup- 
posed, an expansion of it. 

At length Harvey was able to give 
a full account of the whole matter 
(Fig. 33). To begin- with, he said the 
heart has four chambers, the right and 
left auricles, RA and LA, and the 
right and left ventricles, RV and LV. 
The right auricle opens into the right 
ventricle, and the left auricle into the 
left ventricle, and both openings are guarded by valves so 
arranged that fluid may pass from the apricles to the ven- 




tricles but not from the ventricles to the auricles. There is 
nothing in the way of a communication between the right and left 
sides of the heart at all, porous or otherwise. The ventricles 
have very thick muscular walls, and are therefore able, on con- 
traction, to squeeze out their contents with considerable force. 

From the left ventricle arises a large artery known as the 
aorta, A, which presently divides into two branches, the 
smaller branch passing upwards to break into many still smaller 
vessels (not shown on the figure) which supply the head and 
neck, the larger one similarly dividing into secondary branches 
to supply the trunk and lower limbs. Harvey failed to dis- 
cover what became of the blood when it had reached the finest 
visible ends of the arteries. That was not made out until some 
years had elapsed, 

Harvey next saw that the blood from the head and neck 
and from the rest of the body was carried back to the heart 
by two large veins (VC, VC') and poured into the right auricle, 
from which it passed to the right ventricle. Harvey noticed 
that the blood which left the heart by the aorta was bright 
scarlet, but that what came back by the veins to the right 
auricle was dull crimson. From the right ventricle the blood 
was then pumped by a large artery called the pulmonary 
artery (PA) (L&t.fiulmo^lung) to the lungs, where, apparently, 
it regained its bright scarlet colour, and was carried back from 
the lungs by a pulmonary vein (PV) to the left auricle, from 
which it passed into the left ventricle and from which it started 
once more on its long journey. 

It will be noticed that Harvey left two important questions 
unsolved, the first an anatomical one What came in between 
the extreme ends of the arteries and of the veins in the body 
and in the lungs ? and, second, a physiological one What was 
the meaning of the change in the colour of the blood after it 
had passed through the body and after it had passed through 
the lungs ? The first question could not be answered until 
a new instrument, the microscope, had been invented, and the 
second awaited the determination of the relation of the living 
organism to air, and especially to that part of it which Mayow 
found was essential to life. 


The Microscope. To a Dutch optician, named Jansen, 
belongs the credit of having discovered the principle of the 
compound microscope. In its simplest form (Fig. 34) it con- 
sists of a lens, L, nearest the object to be examined, which 
makes a picture, F, inside the tube of the microscope, and 
a second lens, E, next the A 

eye, which magnifies the image. 
Our modern microscopes are 
often very complex instruments, 
for both the object glass and 
the eyepiece are composed of 
several lenses, and, in order to. 
see through an object, there is 
fixed to the stand a platform or 
stage, P, on which the object is 
placed. A hole in the stage 
permits light to be reflected 
from a mirror, M, swinging be- 
low the stage, so as to reach 
the eye through the various 
lenses in the tube. There are 
also screws by which the tube 
may be adjusted to suit the 
magnifying power of the lenses 
and the eye of the observer. 


The compound microscope has now become one of the most 
valuable instruments the biologist possesses, and is to him 
what the telescope is to the astronomer. 

The Royal Society. At this point we must take note of 
one event that took place about the middle of the seventeenth 
century that affected not only biology but all the sciences. 
This was the foundation of what was called the <f Invisible 
College/* a sort of club or society composed of those who were 
interested in science and in the various questions that were 
being discussed by the learned men of the day. The head- 
quarters of the society was at a house in the centre of London, 
called Gresham College, after a famous London merchant, Sir 
Thomas Gresham, who bequeathed it and money for its support 


to the Mercers' Company, on condition that courses of instruc- 
tion should be given there on certain scientific subjects. This 
little band of workers included some of the most famous men 
of the century, such as Boyle, Mayow, Huygens, Halley, Newton 
and many others. After the Restoration, the King was induced 
to take an interest in their work, and at length he officially 
recognised it by giving it a Charter, after which, in 1662, it 
became the most important centre for the discussion of scienti- 
fic questions in Great Britain, if not indeed in the world, under 
the title of the Royal Society. 

The Discovery of the Cell HOOKE. Robert Hooke, who 
had been assistant to Boyle when he was working at the com- 
pressibility of gases, was professor of geometry at Gresham 
College, and also had charge of the setting up of experiments 
for the Royal Society. He was keenly interested in the 
microscope, and made one of his own which he used constantly. 
He cut thin slices of all sorts of bodies, both vegetable and 
animal, and, with the aid of his microscope, discovered that 
they were " all perforated and porous, much like a honey 
comb/' and to these pores he gave the name " cells/' He 
published his observations in 1665 in a book which he entitled 
" Micrographia/' or " little pictures/' 

Capillaries and Blood Corpuscles MALPIGHL Meanwhile, 
in Italy, there lived a very distinguished biologist called Mai- 
pighi, who was professor first at Pisa and then 
at Bologna. He also had the assistance of 
the microscope, and was able to solve the pro- 
blem of the connection between the visible ends 
of the arteries and veins, by showing that they 
were united by an exceedingly delicate network 
of very fine tubules caEed " capillaries/* These 
he saw perfectly clearly in the transparent lung 
of the frog. He also showed that the blood was 
composed of a colourless fluid in which floated 
FIG. 35. BLOOD myr i a( }s of rounded red discs, the " blood cor- 


puscles " (Fig. 35). Each corpuscle is a biconcave 

disc, 6, about ^-^ of an inch in diameter, pale reddish-yellow 
in colour, but scarlet to crimson when seen in mass. Among them 


may be seen " white blood corpuscles/' c, discovered long after 
Malpighi's time, but these are relatively few in number, two 
or three for every hundred of the red ones. 

Malpighi also made many other important discoveries in 
the structure of the human body, such as the different layers 
of the skin and the constituent parts of the kidney, in both 
of which certain regions are still known by his name. He also 
studied the lower animals, and wrote a full account of the life- 
history of the silk-worm, which was published by the Royal 
Society, of which he was a foreign member. Finally, he 
examined the structure of plants, and wrote a long paper, or 
rather book, on the subject, which was also published by the 
Royal Society. In this latter domain he was, however, antici- 
pated by another biologist, this time an Englishman, Nehemiah 

The Anatomy of Plants GREW. Nehemiah Grew, who 
was born in 1641, was a physician, first in Coventry and later 
in London, but he appears to have spent most of his time in 
studying the anatomy of plants. The book in which he pub- 
lished his researches was caUed " The Anatomy of Plants 
begun/' and was profusely illustrated. Both he and Mal- 
pighi made attempts at explaining the functions of the struc- 
tures they described, and sometimes they were fairly right, 
but in other cases entirely wrong. For instance, both of them 
held that sap was pumped through the vessels of the wood by 
a sort of rhythmical pulsation or " peristalsis/' as it was called. 
No doubt they hoped to find something in the plant that 
would correspond to the circulation of the blood in the animal, 
but seeing that there was no heart in the plant, they invented 
the rhythmical squeezing of the wood vessels to take its place. 
Not long afterwards another biologist, Stephen Hales, showed 
that no such squeezing took place, and that there was no 
circulation in the plant comparable with that demonstrated 
by Harvev in the animal. 

The Discovery of Stomata and Chlorophyll. Both Malpighi 
and Grew recognised the presence of minute pores especially 
on leaves, the stomata (Fig. 36), which they correctly thought 
were intended to allow of the escape of superfluous water in 




FIG. 36. STO- 


the form of vapour, or to admit air. Grew paid some attention 
also to the green pigment in plants which we know as " chloro- 
phyll/' and was the first to extract it from 
leaves by the aid of olive oil. What it was for 
he had, of course, not the vaguest idea. He also 
noted how certain plant organs, such as tendrils, 
twined round supports, and how the leaves of 
other plants closed together at night, but the 
explanation he gives is a purely mechanical 
one and very far from the truth. Both Grew 
and Malpighi watched the germination of seeds 
and traced the stages through which they went, 
but the earliest features in the development 
of the embryo could not be made out with the 
apparatus they possessed. 
Biogenesis and Abiogenesis LEEUWENHOEK. In Holland, 
the birthplace of the microscope, there was born in 1623 a man 
called Anthony van Leeuwenhoek, who took a keen interest in 
the microscope. He was well off, and thus could afford to 
spend time and money on his hobby. He both made and col- 
lected microscopes; indeed, it is said that he possessed no fewer 
than 274 of them ! But he made use of them also, and to 
some purpose. He traced the capillaries in the tail of the 
tadpole and in the web of the frog's foot, and his description 
of them is even better than that given by Malpighi. His most 
important discovery, however, was of the minute animals we 
now call " Protozoa/' but which he called "animalcules," and 
of bacteria or microbes, which he found in stagnant water. 
His drawings are so accurate that'modern biologists can actually 
name the organisms he figured. 

It had long been held that various kinds of living things 
could arise from dead matter, and all sorts of ridiculous stories 
were accepted as true such, for example, as the account 
given by the Belgian chemist, Van Helmont, of how he obtained 
mice from cheese wrapped up in soiled linen. 

It was generally supposed that maggots might arise spon- 
taneously from putrefying meat, until, in 1668, Redi, an Italian 
doctor, showed conclusively that the maggots arose from eggs 



that had been laid in the meat by flies. Now, however, that 
Leeuwenhoek had discovered these exceedingly minute creatures, 
the animalculae and bacteria, the suspicion arose afresh that 
they might have sprung from dead material. The theory that 
every living thing springs from another precedent living thing 
of the same kind is now called " biogenesis " or " life origin/ 1 
and the contrary view, that living things may arise from non- 
living matter, is called " abiogenesis " or "non-life origin." 
After careful experiment, Leeuwenhoek decided in favour of 
biogenesis, and in this view he is supported by all recent 
authorities on the subject. 

That Leeuwenhoek was correct in his conclusion may be 
proved in the following way. Prepare some beef extract and pour 
some of it into two test-tubes (Fig. 37), and plug one of them, 
A, with cotton wadding. Heat both of them 
in a pan of boiling water for at least half an 
hour, and leave them to cool. In the course 
of a few days the fluid in B will have be- 
come turbid, and will give off an unpleasant 
odour, but that in A will remain clear. With 
the aid of a powerful microscope the turbid 
fluid will be found to be swarming with ex- 
ceedingly minute organisms, while the fluid 
in A will contain none. How is this ex- 
plained ? Bacteria are the smallest living 
organisms known and occur freely, and 
especially in air. Most of them are killed 
by prolonged boiling, but the boiling must 
be thorough. Some bacteria cause putrefac- 
tion in such food materials as beef-tea, but when the test-tubes 
are boiled all the bacteria already there are killed. When the 
tubes cool more bacteria from the air fall into B, but they cannot 
enter A because of the plug of cotton which has been sterilised 
by the passage of steam through it during boiling. Hence the 
*fluid in A remains good, while that in B goes bad. If the 
bacteria were really derived from the (dead) meat extract, there 
is no reason why they should not have appeared equally in 
both tubes. 

FIG. 37- 


There are two more names that we associate with progress 
in the science of biology in the seventeenth century, both of 
them clergymen, but with very different outlooks on life, and 
very different tastes in research. The one was the Rev. Stephen 
Hales and the other the Rev. John Ray. 

The Foundations of Plant Physiology HALES. Stephen 
Hales was a student of divinity at Cambridge at the end of the 
seventeenth century, when Newton was professor there, and 
perhaps he came under the influence of that great man; at all 
events, he became thoroughly saturated with thescientific spirit, 
and learnt as much about physics and chemistry as he did 
about theology. In 1708 Hales was appointed to a curacy at 
Teddington on the Thames, where he remained for the rest of 
his life, and where all his scientific work was done. His results 
appeared in a volume called ' ' Statical Essays/' part of it in 1727, 
the year Newton died, and a second part six years afterwards. 

It will be remembered that Grew believed in a circulation of 
sap in plants, something like that of the blood in animals, and 
fancied he saw the sap being pumped up the vessels of the 
wood by a sort of rhythmical squeezing. Hales, by very 
ingenious experiments, convinced himself that the sap did not 
circulate, but only ascended, and that there was no pulsation 
in the wood vessels, although there was a pressure that varied 
from time to time according to changes in temperature, light 
and darkness, seasons of the year and other conditions. This 
continuous but varying stream ascended from the roots to the 
leaves where the surplus water was exhaled as vapour through 
the stomata (Fig. 36). He measured the pressure exerted by 
the ascending sap and the rate at which it flowed. What 
caused it to rise ? He was not so confident in answering this 
question, but he thought that the water was forced upwards by 
what he called " root pressure " or vis a tergo (a force from 
behind), carried onwards by capillarity, or the tendency liquids 
have to rise in very narrow tubes, and finally by the pull ol 
evaporation at the leaves, the vis a fronte, or tug from the 
front, so that more water must ascend to replace what had 
been given off. 

Hales made his experiments on sap pressure by means oJ 


a simple instrument called a "manometer'' (Fig. 38), which 
consisted of a twice bent U-tube, one leg of which was firmly 
attached to the stem of the plant, while the other bend of the 
U contained mercury. At first the mercury will be at the 
same level in both legs, but as the sap exudes from the stem 
it will press the mercury down in the nearer leg and cause it 
to rise in the other, and any variations in the heights of the 
mercury will indicate corresponding variations in 
the pressure of the sap. Hales attached these 
manometers to several branches of the same tree, 
and so was able to observe the variations of the 
pressure in different parts under different con- 

Another point that Hales made out was the 
pathway by which the sap ascended. If a branch 
of a tree be cut across, three prominent regions 
may be recognised a pith in the centre, then 
a ring of wood, and outside that a ring of what 
is popularly called " bark." If a young branch, 
still attached to its parent stem, be selected and 
a ring of " bark " down to the wood be cut off 
it, preferably just below the bud, the leaves 
beyond the ring do not wither, showing that the sap is still 
ascending by the wood, while just above the ring the bark 
region begins to swell. This points to the fact that something 
is descending via the bark, for its further downward passage 
is stopped by the removal of that layer; the bud above the 
ring, being better nourished than usual, opens before its 

Hales's book, though written before the composition of air 
was accurately known, and while chemistry was still in its 
infancy, was a wonderful record of careful experiment; while 
the results he obtained, and the sound deductions he drew 
from them, entitle him to rank as one of the foremost biolo- 
gists of the seventeenth century. 

Classification of Animals and Plants JOHN RAY. Ray 
was a biologist of a very different stamp. He was in no 
sense an experimenter like Hales, and took little or 

FIG. 38. 



interest in the life of the plant or animal; he was, on the 
other hand, an enthusiastic collector and classifier. While at 
Cambridge he had as a pupil a young man called Willughby, 
who possessed what Ray had not, abundant private means. 
For several years the two men travelled over much of the 
Continent, collecting plants and animals, and they continued 
to work together on their return, until Willughby died in 
1672. In his will he left Ray a pension and funds to complete 
the work they had begun together. As originally schemed out, 
Willughby was to have done the animals and Ray the plants, 
but in the end Ray did far more than his share, although he 
gives Willughby more than ample credit for what he had 

Classification has changed so completely from what it was 
in Ray's time that his attempts are only of historical interest. 
The first volume published treated of quadrupeds, or four- 
footed animals, which Ray divided into those that laid eggs 
from which the young animal arose e.g., a frog and those 
whose young were born alive e.g., a cow. The first group 
was called " oviparous/' the second " viviparous." The vivi- 
parous animals were next divided into groups according to the 
number of toes they possessed, and then after the number and 
nature of their teeth. Birds were either inhabitants of land 
or of water, and their subdivision rested on the nature of their 
beaks and claws, the length of their legs and the kind of food 
they ate. Fishes were classified very much as they are at 
present. Insects were grouped into those that went through 
changes of form during their lives i.e., caterpillar, chrysalis, 
butterfly or moth and those that developed directly without 
these changes. Ray made no serious attempt at describing or 
grouping the host of lower animals, such as worms, sponges, 
jellyfish, and so on; indeed, very little was known about these 
forms until the beginning of the nineteenth century. 

The best part of Ray's book was the volume on plants. He 
started by separating off all the lower plants such as ferns, 
mosses, mushrooms, moulds and seaweeds as " imperfect " 
plants, because they had no flowers, and by far the greater part 
of his book deals with " perfect " plants i.e., those with 


flowers. The so-called " imperfect " plants are, of course, no 
less perfect than the others, but their real points of difference 
were not made out until well through the nineteenth century. 
No one bothered about them; the flowering plants were those 
that caught the eye and were of the greatest service to mankind, 
and therefore deserved the most attention. 

Ray divided the " perfect " plants into those that showed 
two primary leaves in the seedling and those that showed only 
one. The first he called dicotyledons e.g.> wallflower, bean, 
or rose and the second, monocotyledons e.g., lily or grass. 
These names are still in use, but Ray's subdivisions of these 
two groups have long since been discarded. It is remarkable 
how an error may persist if only it has a great name behind it. 
Ray divided dicotyledons into those with " simple " flowers, 
like a buttercup or a rose, and those with " compound " 
flowers, like a daisy. As a matter of fact, a daisy is not a 
flower at ah 1 , but a closely packed group of flowers or inflor- 
escence, and the real flower of the daisy is just as simple or 
as complex as that of a buttercup. It is smaller, it is true, 
but that is all. Even the first botanist, Theophrastus, who 
lived nearly 2,000 years before Ray, did not make that mistake, 
for he points out quite clearly that a " capitulum " or " little 
head " of a daisy or dandelion consists of many small flowers, 
all arising together at the top of a common axis. 




AFTER realising the tremendous change in the outlook on 
science in general and in astronomy in particular that followed 
Newton's great discovery of universal gravitation, it is not 
astonishing that there were many who believed that little was 
left to be discovered, although Newton himself said that the 
immense ocean of truth still lay unexplored before him. 

The Velocity of Light. Roemer, towards the end of the 
seventeenth century, had shown that light took about sixteen 
and a half minutes to travel from Jupiter to the earth. He 
made his calculations after a study of the eclipses of Jupiter's 
satellites, so that it seemed important to obtain some accurate 
information about those attendants on the greatest planet of 
the solar system, when, for instance, they were predicted to pass 
into his shadow as seen from our earth, and when they actually 
did so. This question of the velocity of light, and the related 
one of the distance of the stars, greatly interested a young 
man called James Bradley, who was born a few years after 
Newton's " Principia " was published. Young Bradley was 
reared in what might be termed an astronomical atmosphere, 
for he spent much of his youth with an uncle who was himself 
a distinguished astronomer. Indeed, uncle and nephew made 
the first attempt at an accurate measurement of the distance 
of the sun from the earth, rather a difficult task when we think 
of the crudity of the instruments with which they worked. 

In 1722 Bradley was elected professor of astronomy at 
Oxford, and he at once set to work on the transit of Mercury 
which was due to take place in the following year, and also on 
tracing the orbit of a comet that Halley had just discovered. 




sp , 


It should be remembered that these observations were made 
with a refracting telescope, which consisted of an object glass 
with a focus of over 200 feet placed at the end of a long pole. 
. The measurement of the distance of the fixed stars was an even 
more difficult problem, and it may be as well to try and under- 
stand how this question is approached before looking at 
Bradley's efforts to solve it. 

Stellar Parallax. Sit in front of a window and fix on some 
object say, a church steeple some distance away and align 
it on a piece of paper gummed on the window-pane; then, 
keeping the body steady, bend the head slightly to the right; 
the paper spot appears to shift to the left (or to the right if the 
head be bent to the left) . Push back the chair from the window 
as far as possible and repeat the observation. The spot will 

still appear to shift to the left 
1 or right with reference to the 
steeple, but the shift will not 
be so great. In short, the far- 
ther off the two objects are the 
less will be the apparent move- 
ment of the nearer one. 

Now the stars are at stupen- 
dous distances from us, but 
these distances vary. Suppose 
we select one star and watch 
it in relation to another star 
much farther off, and then move 
say, from London to Edin- 
burgh about 400 miles. Will 
there be any apparent shift ? 
If there be none, we may con- 
clude that both stars are im- 
mensely far away. But 400 
miles is a mere trifle when we 
are dealing with astronomical 
space, so for our purpose let us fix on two positions on opposite 
sides of the globe. Even then the two stars show no apparent 
shift. The only thing left is to take, as our base, opposite points 



of the earth's orbit i.e., about 190,000,000 miles apart. The 
earth revolves round the sun in an ellipse, so that in mid- 
winter the earth might be at W (Fig. 39), and looking for our 
bright star we find it at X. If we continue the line W X, the 
end of it points to another far more distant star, W. As the 
earth travels in its orbit, it reaches SP in springtime, and, 
again, looking at X, we find the line SP-X points to SP' in the 
sky. Similarly when we are at SU in summer, SU-X points to 
SU', and when at A in autumn A X points to A'. Thus the 
star X appears to describe a tiny ellipse in the sky corresponding 
to the ellipse the earth has described round the sun during the 
year. The astronomers call this a " parallactic ellipse/' parallax 
meaning a slight alteration or deviation. Put more simply, 


if E (Fig. 40) be the earth revolving round the sun, S, and X 
be the star, then the angle made by the two lines drawn from 
X to either end of the radius of the earth's orbit, E S, is called 
the star's " parallax." Obviously the farther off X is the 
smaller will be the angle E X S, until, when X is at an infinite 
distance, the angle will be zero and the two lines E X and S X 
will be parallel. To measure angles so minute requires not 
only the very highest mathematical skill but also the very 
finest instruments. 

If the star we are watching be farther away than X, say 
at Y (Fig. 39), we shall find it also describing an apparent 
ellipse in the sky, but a much smaller one, so we conclude that 
the nearer the star is to us the larger will be the parallactic 
ellipse, and the farther away it is the smaller that ellipse will 
be. How small these ellipses really are may be judged by the 


fact that, if one be standing in the centre of a circle whose 
circumference is two miles distant, a penny placed on the 
circumference would cover the largest parallactic ellipse we 
know ! No wonder that the problem baffled the very greatest 
observers until well into the nineteenth century, when astrono- 
mical instruments began to be sufficiently refined and accurate 
to enable such very delicate observations to be made. Yet 
this was the problem Bradley set out to solve. 

Aberration of Light. The first thing to do was to select 
a star, and Bradley chose one called " Beta Draconis," chiefly 
because it passed over the zenith at Oxford, and so enabled 
him to disregard any refraction of the stellar rays in their 
passage through the earth's atmosphere. He started his 
observations in December, 1725, and expected to see the star 
move to the north, but it did not ! It moved to the south, 
and kept on doing so until March, 1726, when it was 20 seconds 
south of where it had been in December. In April it began 
to move north again, and in June reached the zenith, but it 
did not halt there; it went on northwards until it was almost 
as far north of the zenith as it had been south in the previous 
March. Then it turned south once more and regained its old 
place, just one year after it had begun its curious journey. 
Here was an entirely new kind of movement which was obvi- 
ously not parallactic at aU. Bradley at once gave up trying 
to determine the distances of the stars, and switched off on 
to this new problem. He began by watching the behaviour 
of other stars, and soon found that they all showed the same 
erratic movements to a greater or less degree. 

In order to explain the phenomenon, he argued that since 
light takes a certain time to travel from a star to us, if the 
earth were at rest the ray would reach us in a straight line. 
But the earth is not at rest, it is moving in its orbit at the rate 
of eighteen miles per second, so that when a star is seen through 
a telescope we do not see where it is, but where it was some- 
time before. This is what is called the " aberration " or 
" wandering " of light, and this discovery at once brought 
Bradley into the forefront of the astronomers of his day. 

Soon afterwards Bradley was created Astronomer Royal 


in succession to Halley, and signalised his appointment by 
making another important discovery. He found that when 
the apparent movement of the star was complete it did not 
return precisely to the same spot it had occupied in the previous 
year. Since the days of Hipparchus, more than a century 
before our era, it had been known that the north pole of our 
earth describes a small circle in the heavens. Bradley's new 
discovery was that the circle was not a perfectly smooth curve, 
but a wavy one, each wave taking nineteen years to form. 
To this curious movement of the earth's pole was given the 
name of " nutation," or " nodding/' 

SIR WILLIAM HERSCHEL. While Bradley was working at 
the problem of the aberration of light, there was born in 
Hanover a child who was destined, in time to come, to eclipse 
him entirely, although his rival spent his early days playing the 
oboe in a regimental band. This was William Herschel. He 
may have been a good musician, but he was not much of a 
soldier, for he deserted from the army after his first battle. 
Had he been caught he would probably have been shot, and 
the Hanoverian Guards would have lost an instrumentalist 
and science would have lost, perhaps, the greatest astronomer 
of all time. Anyhow, young Herschel escaped to England, 
and, after filling various musical posts, took up his abode in 
Bath, where he taught music on week-days to the inhabitants 
of that fashionable spa, and played the organ on Sundays in 
the Octagon Chapel. 

It was during these early days he was not yet thirty 
that he devoted his leisure hours to the study of mathematics 
and astronomy, and brought over from Germany his brother, 
Alexander, who was an engineer by profession, and his sister 
Caroline, who proved of immense service to him in the years 
that followed. 

Nothing less would satisfy him than to see with his own 
eyes the wonders of the heavens that others had seen and 
written about, so he set about making a telescope better than 
any he could afford to buy. After many attempts, in which 
he had his brother's help, he at last turned out an instrument 
that satisfied him, and with it began to carry out what 


astronomers call "sweeping the heavens" i.e., exploring 
methodically every field of view the telescope presented. 

Newton, it may be recalled, had given up the idea of a 
refracting telescope, and invented one where the light from 

the stars was reflected from a 
concave mirror, the rays being 
focussed half-way down the tube 
and transmitted by a flat mirror 

FIG. ^.-HBKSCHBL'S TELESCOPE. to an eyepiece set in the side 

of the tube (Fig. 28). Herschel 

greatly improved on this by tilting the concave reflector and 
so directing the rays sideways. They could thus be seen by 
the eye directly (Fig. 41). 

Discovery of Uranus. One night, on March 13, 1781, to 
be precise, he was " sweeping the heavens " as usual, and 
noticed in one of the constellations, called " Gemini/' or the 
" Twins," a new body that differed from all the other luminous 
points beside it in showing itself as a disc and not as a mere spark 
of light. He thought at first it must be a comet, but soon 
decided that it was not. The only other thing it could be was 
a new planet, a new member of the solar family. Five planets 
had always been recognised from the earliest times viz., 
Mercury, Venus, Mars, Jupiter and Saturn. Who first dis- 
covered them we do not know, and certainly no one ever 
dreamt of looking for a sixth. It may easily be imagined with 
what astonishment scientific men learnt that an unknown 
music master in Bath had found a new child for Father Sol, 
and no puny infant either, for the newcomer was more than 
sixty times the bulk of the earth. It is true he was rather 
a shy youngster, for he followed his own course, nearly twice 
as far away as his nearest big brother, Saturn. The " little 
stranger " was christened " Uranus." 

As very often happens, the arrival of the new baby com- 
pletely upset Herschel's household. He was summoned to 
Windsor by the King, who created him his own astronomer, 
and provided him with a house and salary. His devoted 
sister Caroline and all the lares et penates were presently housed 
in a new abode, and Bath saw her organist and music master 


no more. Herschel had sacrificed a much bigger income for 
the paltry 200 a year that the King aDowed him, but he did 
not grudge the loss, for now he was able to devote himself 
entirely to astronomy, and to enjoy complete freedom from 
the treadmill of music lessons, pump-room concerts and choir 
practices. It is true he had to eke out his slender income by 
making telescopes for sale, until an influential friend managed 
to get the King's ear and induce His Majesty to provide the 
funds for a new and much more efficient telescope for Herschel's 
own use. This instrument ultimately cost over 4,000. 

Herschel's first exploit with the new telescope was to 
provide " Uranus " with a couple of moons to bear him com- 
pany in his far-off wanderings, and to add two more to the 
five Satellites that Saturn was known to possess. 

Then another important event took place in his own 
family this time he married a lady who not only thoroughly 
sympathised with her husband's work, but, by her wealth, 
relieved him from all monetary worries. After his marriage* 
he moved into a much more commodious house at Slough, 
where he lived for the remainder of his life. 

Nebulas and Double Stars* The amount of work Herschel 
got through during his life at Slough was prodigious. He 
discovered over 2,500 nebulae and star clusters, and in one 
region of the " Milky Way " he suddenly met with an exceed- 
ingly dark area, as if the whole stellar universe had been 
pierced by a gigantic hole. " Surely there is a hole in the 
heavens !" he exclaimed to his sister, but he had no explana- 
tion to offer of this extraordinary phenomenon. Modern 
astronomers have come to believe that these dark patches are 
due to gigantic clouds of cosmic dust, blocking off the light 
of the stars beyond them. 

Herschel catalogued over 800 {t double stars/' as they are 
called. In some cases he noticed that in these paired stars 
one was much fainter than the other, which, for that reason, 
he thought must be much farther away. He tried to deter- 
mine the parallax of the brighter of the two as against the 
other, but he could discover no shift after an interval of six 
months. But he did find that both were as large as, if not 


larger than, our own sun, and revolved round each other, just 
as the thumbs may be made to revolve when the fingers are 
interlaced--'' twiddling," as it is often called. 

Drift of the Solar System. Perhaps the most sensational 
discovery Herschel made was that our whole solar system, the 
sun with all his attendant planets and their satellites is moving 
through space. Where to? Even that profound question 
Herschel was able to answer. If we should happen to be at 
sea at night and approaching a harbour, the entrance to which 
is marked by two lighthouses, while still at a great distance 
off the two lights may appear only as one, but as we gradually 
approach the harbour the single light resolves itself into two, 
at first quite close together, but slowly diverging, until, when 
we near the entrance, they stand right and left. 

Now in one very far distant constellation called " Hercules," 
Herschel, after prolonged watching, noticed that the intervals 
between certain stars appeared to be slowly widening, and 
that observation suggested to him that our little system was 
graduaUy travelling towards the diverging stars in " Hercules/' 
The story is not quite so simple as it looks, but it was a bold 
guess on Herschel's part, and our modern astronomers have 
nothing to say against it. 

Think what a change had taken place in the conception of 
the heavens since the days, only 150 years before, when Galileo 
was content with proving that the earth went round the sun, 
to his own satisfaction at least, if not to the minds of those 
who were too ignorant or too prejudiced to see " the vision of 
the world and all the wonder that would be." Herschel had 
truly founded a " Science of the Stars/* 

SIR JOHN HERSCHEL. In 1792 a son was born to Sir William 
Herschel, who many years *ater became also a great astronomer, 
though overshadowed to a large extent by his more distin- 
guished father. Sir John Herschel, as he ultimately became, 
followed closely in his father's footsteps, and " swept the 
heavens " of the southern hemisphere as Sir William had done 
the northern, living for several years in South Africa for the 
purpose. In addition to charting over 2,000 double stars he 
made a careful study of two enormous masses of what looked 


like luminous vapour which were visible only in the southern 
skies, known as the " Magellanic clouds/' and which he found 
to be composed of groups of stars of all sizes, nebula, and a 
general luminous dust, the nature of which his i8-inch reflector 
was unable to reveal. 

One of Sir John's most useful pieces of work was the writing 
of a general account of all that was then known about the 
heavens in language that anyone could understand, and thus 
he enabled the general public to learn something of the wonders 
that had been discovered. 

After the enormous strides that had been made in our 
knowledge of the heavens during the seventeenth and eighteenth 
centuries, it is not to be wondered at that there were not a few 
philosophers who longed to have some reliable theory that 
would explain how all these systems of worlds came into being. 
They must have had a beginning, and if so, what was that 
beginning like ? Would they last for ever, or would there be 
an end some day, and what kind of an end would it be ? 

Cosmogenesis LAPLACE. There was one great genius, a 
Frenchman, who was younger than Sir William Herschel, and 
who made an effort to answer these questions. His name was 
Pierre Simon, the son of a small farmer near Honfleur, on 
the Seine, opposite Havre. Towards the end of his life he 
became a very distinguished man of affairs under the great 
Napoleon, and was ultimately ennobled as the Marquis de 
Laplace. Laplace is best known by his two famous books, 
the "Systeme du Monde/' or the "Theory of the Earth/' 
published in 1796, and the "Mecanique Celeste/' or the 
" Mechanics of the Heavens/' which appeared in 1799. Since 
the " Mechanics/' as a distinguished modern astronomer has 
said, is " one of the most difficult books to understand that 
has ever been written/' we shall leave it severely alone. No 
one need attempt to read it who has not been naturally 
gifted with a mathematical brain, highly polished by constant 
use. The other book was much simpler both in its matter 
and its style; and in it Laplace sketched out the story of our 
solar system as he imagined it, a story which we always call 
Laplace's "Nebular Theory/' 



Without troubling ourselves with the mathematics of the 
question at all, we shall have no difficulty in grasping the theory 
after having mastered the general construction of the solar 
system, as we know it now. Fig. 42 is a plan of the solar 
system from the sun to the orbit of Saturn, the orbits of the 
various planets being approximately at their relative distances 
from each other. The numbers below the initial letters of the 
planets are their mean distances from the sun in millions of 
miles, any fraction over half a million being taken as one. On 
studying the figure it will be seen that there is a wide gap be- 
tween the orbit of Mars and that of Jupiter, and the astronomers 
of the eighteenth century often speculated on the possibility 

s A 



Nos. in millions of miles of distance from the Sun. S, Sun; ME, Mercury; 
V, Venus; E, Earth; MA, Mars; MP, Minor planets or asteroids; J, 
Jupiter; SA, Saturn. 

of there being another planet circling in the vacant space. 
One of these astronomers, a contemporary of Herschel, called 
Bode, noticed a peculiar fact. In the simple sequence of 
numbers o, 3, 6, 12, 24, 48, 96, each (save the second, of 
course) being double the number preceding it, when now 4 is 
added to each of these numbers the sequence 4, 7, 10, 16, 
28, 52, 100 is obtained. Comparing these figures with those 
of the relative distances of the planets from the sun viz., 
Mercury 3-9, Venus 7-2, Earth 10, Mars 15*2, Jupiter 52-9, and 
Saturn 95-4 it will be noted that they correspond very closely 
with the sequence 4, 7, 10, etc., with one exception there is 


nothing to represent the sequence number 28. This sequence- 
relationship came to be called " Bode's Law/' 

The Asteroids. Was there a planet missing ? Search was 
made, but in vain, for several years until in 1801 an Italian 
observer, named Piazzi, discovered a tiny little object that 
moved like a planet, but whose diameter was only 485 miles, 
or about a quarter of that of our moon. This minute body 
was called " Ceres." If the classical deity after whom this 
little planet was named be the goddess of plenty, her name- 
sake in the heavens was to be the forerunner of an abundant 
crop of brothers and sisters, for at the beginning of the twentieth 
century we knew of well over a thousand of these " asteroids/' 
or tiny planets, whirling round the sun in a majestic zone, 
between the orbits of Mars and Jupiter. 

Relative Sizes of the Planets. So much for the relative 
positions or distances of the planets from the sun; there is left 
for consideration their re- 
lative sizes. Fig. 43 repre- 
sents their circumferences 
drawn roughly to scale, one 
within the other. The dotted 
line represents the relative 
size of Neptune, not dis- 
covered until long after the 
time of which we are speak- 
ing. Of course, we cannot 
represent the sun, for he 
would require a circle nearly 
two feet in diameter if drawn 
on the same scale. The earth 
is about 8,000 miles in dia- 
meter, Jupiter 86,500 miles, 
while the sun is more than 
ten times that viz., 866,000 
miles, which means that Jupiter is a thousand times and the 
sun a million times the bulk of the earth. Yet, in the days of 
Galileo, people quite seriously believed that this insignificant 
fragment was the centre of creation, and that all the rest was 








manufactured for its benefit ! Such was the colossal egotism 
of the geocentric cosmogony ! 

The Nebular Hypothesis. Let us turn now to Laplace's 
dream of how all this great system of worlds came into being. 
He imagined that the solar system originated in a nebula, i.e., 
one vast cloud of gas, hundreds of millions of miles across, at 
a temperature far, far higher than any electric furnace that we 
have as yet devised can reach, so hot, indeed, that a metal like 
iron could exist only in the form of a vapour. Further, he 
thought of this vast cloud as rotating with a speed which in- 
creased as it contracted by cooling. In the course of eons of 
time, this huge cloud, as it cooled, began to condense, most of it 
into a central denser core, the rest forming a gigantic but still 
gaseous envelope which flattened owing to centrifugal action. 
The latter effect increased as the speed of rotation increased till 
at a critical point matter was thrown off the outer edge of the 
disc-like nebula, and this process led to the detachment of 
separate condensations by repetition of centrifugal effect. 
Circling round the central fire, and each of them, at the same 
time, spinning on its own axis, all these condensations moved 
in the same direction and by repetition of the centrifugal process 
gave rise to secondary condensations or satellites. The sun thus 
spins on its axis once in twenty-six days; the earth rotates on 
its axis once in twenty-four hours, and aE the other planets 
follow suit, all, with only minor exceptions, rotating and re- 
volving in the same direction. 

By and by these condensations began to lose some of their 
heat, and became first liquid and then solid, and formed the 
planets and moons of our solar system, while the great central 
mass, being so vastly larger, retained much of its primeval heat, 
with a semi-gaseous consistence (for it is now only about one 
and a half times as dense as water), and became our sun. On 
that hypothesis the great zone of asteroids between Mars and 
Jupiter would represent a ring of material which, instead of 
condensing into one planet, aggregated into several hundreds 
of smaller ones, comparatively so minute that it has taken 
more than a century for our best astronomers to count them 
aE, if they have completed the tally even yet. 


If this were the true story of the origin of our solar system, 
the same sort of thing may be supposed to occur all through the 
universe, although in our brief span of life we could not hope 
to watch the actual making of another solar system. As a 
matter of fact this brilliant hypothesis is known to be erroneous 
so far as the origin of our solar system is concerned, and Laplace's 
theory may be described as a dream, but it may be a dream that, 
in some form, is a reality in many a yet unexplored corner of 
the " boundless universe'* (see p. 332). 

The Discovery of " Neptune." Let us imagine an onlooker 
sitting in the gallery of a large public hall, watching the move- 
ments of the crowd below. Presently he notices a newcomer 
entering the doorway, who, after a pause, recognises, at the 
other end of the hall, a friend with whom he very much desires 
to talk. On account of the crowded floor he cannot reach his 
friend by pushing forward in a straight line; someone collides 
with him on one side, another on another side, or he turns 
aside for a moment to shake hands with an acquaintance, but 
he gets to his destination in the long run. From his seat in 
the gallery our observer can watch his progress and see for 
what point he is steering, although his path is by no means 
a straight one, as it might be if all the other people in the 
crowd were blotted out. Now let the onlooker imagine that 
he sees only the person he is interested in, and none of the 
other jostling folks, who may be supposed to be wearing the 
" helmet of Orcus " that gave invisibility to anyone who 
wore it. He might very well be puzzled to understand why 
his friend did not move in a straight line without constantly 
wobbling from side to side in such an erratic manner. The 
bearing of this simile will appear presently. 

We have just seen how Herschel discovered, almost by 
accident, the new planet " Uranus/' majestically pursuing his 
long journey of nearly 6,000 million miles round the sun, a 
journey that took him eighty-four years to complete. Of 
course, after the discovery of Uranus the eyes of astrono- 
mers all over the world were fixed on him, in the hope of 
determining his exact , size, his peculiar characteristics if, 
he had any the speed at which he moved, and so on. 


One of the points that had to be decided was his exact path 
or orbit. 

Astronomers had for many years been charting the heavens, 
marking the precise positions of every star that their tele- 
scopes disclosed, so, when Uranus was discovered, it naturally 
occurred to the "Watchers of the Skies" to look up the old 
maps to see whether the position of Uranus had been re- 
corded before, without any astronomer having dreamt that 
it was a planet and not a star. There was one old chart, 
dating from 1690, preserved in the observatory at Greenwich, 
that gave a picture of the heavens in the region where Uranus 
might have been then, and there was one star in particular 
that had attracted the attention of Flamsteed, at that time 
Astronomer Royal, but when it was looked for, behold ! it 
had vanished. It had wandered out of the field. The natural 
conclusion was that this was the new member of the solar 
system that had been discovered by Herschel. Flamsteed had 
seen it several times, but, alas ! he had failed to recognise it 
as a planet. Small blame to him, for another astronomer, 
Lemonnier, had also seen and recorded it in his charts at least 
a dozen times, and yet had failed to identify its real nature. 

When Uranus was welcomed into the circle of our solar 
system, it became of great importance to plot out its course 
in the heavens and see whether it also obeyed Newton's law 
of universal gravitation. This was a very laborious task, for 
no one man could hope to follow the new member of the family 
throughout an entire journey of eighty-four years* duration. 
Still, the observations were made and the orbit of Uranus 
was plotted out, but when this had been done, lo and behold ! 
instead of following the pathway the astronomers had laid 
out for him, he declined to follow it, any more than the visitor 
took the path he was expected to take towards his friend in 
the assembly hall. Was Uranus disobeying Newton's law ? 
When all the persons in the hall wore the " helmet of Orcus " it 
was impossible to see them, but the erratic course the visitor 
followed in his effort to cross the hall could be followed. One 
could only conclude that there must have been something 
enticing or pulling him over to one side or the other. So it 


began to be realised that there must be something disturbing 
Uranus, some boon companion holding out a hand of good 
fellowship as he passed him by. 

Instead of peering through telescopes looking for the un- 
known who wore the " helmet of Orcus," two mathematicians 
collected all the information they could find about Uranus's 
vagaries and, using their mathematical skill, sat down in their 
studies and attempted to find out what the attraction was that 
made the planet deviate from the path he ought to have 
followed. These vagaries were called the " Perturbations of 
Uranus/' and they had perturbed the minds of the astronomers 
of the early years of the nineteenth century very much indeed. 
The two astronomer mathematicians were John Couch Adams, 
a young graduate of Cambridge, and U. J. J. Le Verrier, the 
director of the Paris Observatory. Both started to solve the 
puzzle, each quite ignorant of what the other was doing. The 
story of the struggle for the first place in the race is an 
interesting one. 

Adams began his research soon after he had taken his 
degree in 1843, when he was only twenty-four years of age, and 
two years later he had solved the problem by mathematical 
reasoning alone. He found that the cause of Uranus's wander- 
ings from his proper path must be the pull of yet another 
planet far beyond the orbit of Uranus, which as yet wore the 
" helmet of Orcus," for it had never been seen, or at least 
had never been recognised as a planet. Having worked out 
its orbit and fixed on the spot in the heavens where it was 
most likely to be found, Adams told his story to the then 
Astronomer Royal, Sir George Airy, in October, 1845, and 
asked him to search the heavens for the unknown; but this 
could only be done by comparing the telescopic field with 
a chart of the stars in the same region, and such a chart was 
not available. 

Meanwhile Le Verrier's attention had been drawn to the 
same eccentricities of Uranus, and by June, 1846, he also, 
by mathematical reasoning alone, had decided that some 
disturbing planet must exist. His results, when they were 
published, thoroughly startled the authorities at Greenwich, for 


the position that Le Verrier gave for the stranger was within 
a degree of that which Adams had predicted nine months 
before. That two men, working quite independently of each 
other and on the same material, should have come to the same 
result was felt to be something more than a mere coincidence, 
so Airy asked Challis, at that time professor of astronomy at 
Cambridge, to map out the region of the heavens in which 
Adams had said the unknown planet would be found. Galle, 
the head of the Berlin Observatory, was invited by Le Verrier 
to explore the same region. Galle already possessed what 
Challis was only in the act of making a chart of the star area 
in question. It may be imagined with what anxiety Galle 
unrolled his map and checked off every star in the field of his 
telescope with those in the corresponding section of his map . At 
last one bright uncharted spot was noticed, and on the follow- 
ing night it was still there but it had moved ! There was no 
longer any doubt about it; here was the unknown, who had 
at last taken off his helmet of invisibility. 

A controversy at once arose between the French and British 
astronomers as to which of the two men belonged the credit 
of the great discovery. Now, both sides are content to say 
"honours even/' and to agree in christening this, once sup- 
posed last, addition to the family circle by the name of "Nep- 
tune." We know much more about him now; he takes twice 
as long as Uranus to girdle his parent Sun ; he is a little larger than 
Uranus, but less than half the diameter of Jupiter. As Neptune 
is 2,800 millions of miles distant from the Sun, which is 30 times 
the earth's mean distance, it will be seen that this planet does 
not conform to Bode's Law (p. 82) which requires about 38 times 
the distance. Uranus, however, conforms approximately. 

After the excitement caused by the discovery of Neptune 
had died down, nothing of much importance took place in 
astronomy for some time. She was waiting for her next great 
advance until her sister sciences, physics and chemistry, had pro- 
vided her with new instruments wherewith to probe the depths of 
space. We may therefore make use of the pause to turn to the 
other sciences and see what progress they had made since the 
days when Newton wrote his immortal work, the " Principia." 



At the beginning of the eighteenth century, geology, the 
study of the earth's crust and its partial covering of ocean, 
was in a very backward state. Men like Buffon speculated 
vaguely about the origin and history of the world long before 
enough was known about its structure to justify any con- 
clusions on the subject. The foundations of the science had yet 
to be laid, and fortunately the new century saw the rise of a 
new class of observers who contented themselves with collecting 
data, leaving speculations to the future. 

The Foundations of Geology GUETTARD. One of these 
was a man whose services to the science have been rather 
ignored, perhaps because he was shy and retiring by nature, 
and because his discoveries did not shine out with the bright- 
ness that made a halo round the names of the finders of new 
planets in the solar system. This man was Jean tienne Guet- 
tard, an apothecary in fitampes, a village a few miles from 
Paris. He was born in 1715, and, as a boy, was a very keen 
naturalist, never so happy as when he was collecting plants 
and watching the changes taking place in the rocks and soils 
on which they grew. About that time the science of botany 
was moving forward under the care of Be Jussieu, and, owing 
to his influence, young Guettard was appointed curator of 
the fine natural history collection that had been made by the 
Duke of Orleans. 

During his wanderings over Western Europe in search of 
plants, Guettard soon realised that their occurrence depended 
largely on the nature of the soil and rock in the neighbour- 
hood, and thus he was more and more drawn away from the 
study of the plants themselves to that of the rocks which lay 
beneath them. He became, in short, a mineralogist,, and began 
to record the occurrence of rocks of the same kind in different 
parts of the same country. It should be remembered that the 
very word " geology " was not then in use; it was invented 
some fifty years later by another distinguished observer named 
De Saussure, of whom more anon. 


Guettard, in his study of the rocks, did not fail to note the 
fossils that lay embedded in them, and he was under no mis- 
apprehension as to how they found their way there. He held 
that they were precisely what they appeared to be viz., the 
petrified remains of plants and animals that had lived on the 
surface of the earth at the time these rocks had been formed. 

Geological Maps. Guettard was among the first to make 
a map showing the way in which the different kinds of rock 
were distributed over France, the forerunner of the geological 
maps with which we are so familiar, and issued from time to 
time by the Geological Surveys of different countries. There 
were no political divisions in Guettard's maps; aU he was 
concerned with was the distribution of the same kind of rock, 
whether in France, Germany, Belgium or Britain. His work 
on fossils was particularly important, because he held, and 
rightly, that these remains were the key to the past history 
of the earth. Indeed, he looked on the earth's crust as a vast 
cemetery wherein lay buried the remains of the ancestors of 
the beings now living on its surface. One of his papers bore 
the quaint title, " On the accidents that have befallen Fossil 
Shells compared with those which are found to happen to Shells 
now living in the Sea/' and that title really suggests the lesson 
that modern geology teaches viz., that, in order to know 
what happened in the past, it is essential to study what is 
happening now. Another of his papers was " On the Degrada- 
tion of Mountains effected in our time by Heavy Rains, Rivers 
and the Sea/' and that is precisely what the old Greek philo- 
sopher, Pythagoras, taught more than 500 years before our era. 
Great credit is due to the Frenchman who thus turned men's 
minds from fancy back to fact, from guesswork to reality. 

The Volcanoes of Auvergne. Guettard was the first to 
explain the nature and origin of the strangely shaped mountains 
known as " The Auvergne " in southern France, not far from 
Clermont. One day he was wandering near that region and 
noticed that the milestones were made of a sort of black rock 
which he thought must be of volcanic origin, and, as he followed 
them on his journey, he found that the villages began to be built 
of the same material. Asking where the rock had come from, 


he was told from a quarry some ten miles distant. Sure enough, 
when he got there he found it was a quarry of lava, and with 
great perseverance he at last located the cone and crater of an 
old volcano. Hence he concluded that this quiet pastoral 
country had, in days long gone by, been the scene of tremendous 
disturbances, when glowing mountains belched forth streams 
of molten rock, as Vesuvius and Etna were doing at the 
moment, although he had never seen them. He could not get 


From Geikifs ''Text-Book of Geology," Vol. I., by kind permission of 

Macmillan and Co. , Ltd. 

rid of the old idea, however, that the cause of all this volcanic 
activity was the burning of coal, petroleum and other com- 
bustibles deep down in the earth's crust, and in support of his 
view pointed to the beds of asphalt at Clermont. 

The Nature and Origin of Basalt DESMAREST. Another 
geological pioneer, also a Frenchman and a contemporary of 
Guettard, was Nicholas Desmarest, who was born in 1725, near 
Brienne. His parents were extremely poor, so poor, indeed, 
that had not Nicholas been practically adopted by the Catholic 


Seminary at Troyes, he would never have been educated at all, 
for it is said that he could hardly read when he was fifteen years 
old. After ten years of drudgery spent in teaching what he had 
learnt at Troyes, he competed for a prize offered for the best 
essay on the question whether England had ever been geo- 
logically united to France. He won the prize and, as a con- 
sequence, made the acquaintance of the famous mathematician 
D'Alembert, who in turn introduced him to the Due de Roche- 
foucault, who proved himself a warm friend and patron of the 
young geologist. Owing to the powerful influence of this 

From " Chambers 's Encyclopedia-," by permission. 

nobleman, Desmarest was ultimately made a Director of 
Manufactures, but only just escaped the fate of his benefactor 
in the perilous days of the Revolution. 

When peace returned to the troubled country, Desmarest 
followed in Guettard's steps and wandered over the land, adding 
to his geological knowledge, until he found himself in Auvergne, 
the scene of Guettard's labours a dozen years before. There 
he set himself the task of working out the nature and origin of 
the curious black stone so common in France and other parts 
of Europe, and occurring frequently in erect polygonal columns, 
familiar to us in Britain in the Giant's Causeway and in the 


island of Staffa off the west coast of Scotland, The real diffi- 
culty in interpreting this remarkable formation lay in the 
fact that it was to be found wedged in between layers of rock 
that had undoubtedly been laid down in the sea. He noticed 
that this " basalt," as it was called, from an old name given to 
it by the Roman historian, Pliny, often occurred in isolated 
patches, lying far apart from each other, and the conclusion he 
came to was that all these patches had at one time formed a 
continuous sheet of lava, but had subsequently been cut into 
fragments by the constant erosion of rivers. He thus laid 
still further stress on the doctrine of denudation that is now 
emphasised in every textbook of geology. 

Desmarest was most conservative in his habits. He always 
rose, had his meals, and went to bed at the same hours, and 
never changed the cut of his clothes all his life long. Perhaps 
this extreme regularity in his habits, and the fact that he spent 
so much of his time in the open air collecting material for his 
writings, explain why he lived to the advanced age of ninety, 
without ever having had a serious illness. 

The Geology of the Alps H. B. DE SAUSSURE. Everyone 
who has visited Switzerland or has seen photographs of its 
magnificent scenery can visualise its lofty snow-clad mountains 
with glaciers or rivers of ice sliding slowly but unceasingly down 
the higher valleys, with turbulent muddy streams of ice-cold 
water issuing from under their lower ends. Amid such surround- 
ings was born, at Geneva, in 1740, Horace Benedict de Saussure. 
He must have been a precocious youth, for, after a brilliant 
university career, he became a professor before he was twenty- 
two. His tastes at first lay in the science of botany, but he 
soon branched off into geology and mineralogy, and it was he 
who first christened the science of the history of the earth's 
crust by the name by which we now know it viz., Geology. 

It was impossible for him to live amongst such scenery 
without feeling the lure of the mountains, and to him belongs 
the credit of having been the first to travel over and explore 
them from end to end, and not merely to view them from a 
distance. Listen to his own words expressing the feelings of 
the naturalist who dares to leave the beaten track in the valley 


and climb some of the jagged peaks that soar 10,000 feet or 
more above him. 

" Many a time, the naturalist, when almost within reach of 
a summit on which he eagerly longs to stand, may doubt 
whether he has strength enough left to gain it, or whether he 
can surmount the precipices which guard its approaches. But 
the keen fresh air which he breathes makes a balm to flow in 
his veins that restores him, and the expectation of the great 
panorama which he will enjoy, and the new truths which it 
will display to him renews his strength and his courage. He 
gains the top. His eyes, dazzled and drawn equally in every 
direction, at first know not where to fix themselves. By 
degrees he grows accustomed to this great light, makes choice 
of the objects that should chiefly occupy his attention, and 
determines the order to be followed in observing them. But 
what words can describe the sensations or the ideas with which 
the sublime spectacle fills the soul of the philosopher ? Stand- 
ing as it were above the globe, he seems to discover the forces 
that move it at least, he recognises the principal agents that 
effect its revolutions." 

It is curious to note how De Saussure clings to the old 
ideas about the origin of rocks. There were many geologists 
of his time who held that the granite that formed the backbone, 
so to speak, of the mountain mass of the Alps, was first de- 
posited in the sea and afterwards crystallised, and that the 
sheets of limestone and other strata that lay against the 
flanks of the ridges were formed in that position. Although at 
first De Saussure held such views, he changed his mind later 
on, and admitted that these tilted layers could not contain 
sand and water-worn pebbles unless they had been formed 
horizontally at first and subsequently upheaved. The only 
explanation that would account for such elevation was the old 
one of vast internal volcanic fires, but he rejected it because he 
could find " neither mineral nor stone which might be suspected 
to have undergone the action of these fires/' He began to 
picture a series of rock masses, in part at least deposited in 
successive layers, and then, owing to the contraction of the 
earth's crust, bent into wave-like folds, "as a number of heavy 



carpets laid one on another would be if their opposite ends were 
pushed nearer together/' Occasionally the thrust is so powerful 
that the folds are bent over in the opposite direction (Fig, 46, 
JL, L, T) ; sometimes the rock masses are even broken under 
the strain, and the upper part of the fold slides forwards over 
the under (L). Then over and above all this squeezing and 
folding comes the action of rain, avalanches and glaciers, relent- 
lessly scouring out valleys and scouring the mountain sides, while 
frost is for ever wedging off fragments and boulders of every 
conceivable size and shape to form more and more polishing 
powder wherewith the glaciers and the rivers may grind the 
primeval mountain forms into their present shapes, and deposit 


JL, Jurassic limestone; L, Lias; T, Trias. 

the debris, grain by grain and pebble by pebble, on the floor of 
the ocean. It was this operation that most attracted De 
Saussure's attention, and it is to his vivid description of this 
slow but never-ceasing erosion of the mountains that we owe 
so clear a conception of what we nowadays call " Denudation." 
TheTheory of the Earth HUTTON. AsNewtonwith his law 
of universal gravitation struck the keynote of all further progress 
in astronomy, so James Hutton settled for all time the lines of 
advance in geology, and the text of his sermon was: " What is 
happening now happened in the past/' Hutton was born in 
1726, and was the son of the Treasurer of the City of Edinburgh. 
After taking a degree in medicine, he devoted himself to the 
study of chemistry with the view of becoming a scientific farmer. 
He settled on a farm in Norfolk where he examined chemically 


not only the nature of the soils on which he grew his crops, but 
also the peculiarities of the underlying rocks from which these 
soils were largely derived. In 1754 he returned to the neigh- 
bourhood of his native city and proceeded to cultivate a small 
estate he had inherited from his father. During the next 
fourteen years he studied deeply the various problems that 
were being discussed by the French geologists of the day. As 
more and more of his time was given up to these pursuits he 
felt himself obliged to let his farm and transfer his abode to 
Edinburgh, where he became an intimate friend of the then 
professor of chemistry, Joseph Black. For more than a quarter 
of a century Button patiently worked through all the treatises 
he could find on geological subjects, and checked the statements 
in them by his own observations made in the rich fields offered 
to him round Edinburgh and in other parts of the British Isles. 
Many papers on special points came from his pen, and one of 
these was a general outline of earth structure, which was read 
to the Royal Society of Edinburgh. This society was founded 
in 1783, largely owing to his efforts along with those of his 
friend Black, a society which represented in the Scottish capital 
what the older Royal Society stood for in London. In 1795 
Hutton published his great work called " A Theory of the 
Earth/' only two years before he died. 

Although this work is now regarded as a classic in science, 
it was not a very readable book, for his style of writing left 
much to be desired, and he did not always arrange his facts in 
such a manner as to carry conviction to his readers. Fortun- 
ately for him and for the science he had an intimate friend and 
enthusiastic admirer in the Rev. John Playfair, Minister of 
Benire, near Dundee. Playf air was not only an able clergyman 
of the Church of Scotland, but also a mathematician and 
geologist of considerable repute; indeed, his attainments in 
these subjects were so marked that he became professor of 
mathematics and, later, of natural philosophy, as physics is 
called in the northern universities. Realising the difficulties 
people had in following Hutton's arguments as expounded in 
the " Theory/' he took upon himself the task of writing what 
might almost be called a translation of it which he published 


in 1802, after Button's death, under the modest title of t( Illus- 
trations of the Huttonian Theory of the Earth." Our distin- 
guished modern geologist, not long since dead, Sir Archibald 
Geikie, describes this book as a " consummate masterpiece." 
What Button's views on earth structure were may be most 
easily grasped by a study of Playfair's admirable summary. 

Button's general thesis is that there is " nothing new 
under the sun "; what is taking place today is just what took 
place in days gone by, although perhaps in some respect more 
vigorously. Below the soil on which we grow our crops and 
build our cities lie beds of sandstone, limestone, shale, or gravel, 
all derived from the waste of mountains and lower lands, 
carried down by rivers and deposited on the bed of the ocean. 
We see this going on now, and so it must have been when there 
was no human eye 

to see it. The rocks -^^^^3^^ " . . . 

Limestone with fossils 

Shale with nodules of ironstone 
Shaly sandstone 

, , , , I* , 

we clamber over to- *. 1VJ 

day are only the 

consolidated sedi- "^r^^r^ ThTrTbedded sandstone 

ments from the 
rivers of the past, 

Pebbly sandstone 
Mixture of clay 

solidified by the J^V ;* /-?V rounded stones 

weight of superin- * 


cumbent layers laid (AFTER SlR A GEIKIE } 

down on them from 

age to age. Hence Hutton recognised primary rocks and 
lying over them, secondary, all baked by subterranean heat, 
and squeezed or crushed into cakes or strata. These strata, 
once horizontal (Fig. 47), afterwards became bent, twisted, 
folded and tilted up on end by internal convulsions of nature, 
long eons ago, convulsions which Hutton believed were due 
to some gigantic forces emanating from the molten interior 
of the earth. What these might be he did not venture to 
suggest, holding that geology was not concerned with the 
origin of things, but only with things as they are. 

But there were other kinds of rocks that did not appear to 
have been laid down in beds, and he fancied that these had been 
originally molten and pressed in between primary and secondary 


strata while the bending and folding was taking place, so pro- 
ducing very distinct changes in the stratified rocks into which 
they had been forced. Granite, he thought, was such a rock, 
not deposited in the sea and then crystallised, as some believed, 
but a rock formed in some way by the agency of fire. 

The next important principle that Button emphasised 
was the perpetual wearing away of mountains and hills by rivers 
that carried off fragments, from microscopic size up to stones or 
even boulders, scraping the sides of the valleys, lowering the 
summits of the mountains inch by inch, and spreading all the 
debris over the bed of the ocean. When glaciers were the agents 
they were able to transport on their surfaces much larger 
masses that had tumbled on them from the cliffs above, and 
might carry them for many miles away from their original home, 
where they might be stranded by the melting of the ice, and 
left on a land totally different, geologically, from that which 
gave them birth. These boulders are what we call " erratics," 
or wanderers. This was a subj ect very fully worked out in later 

StratigrapMcal Geology, and Palaeontology W. SMITH 
Towards the end of the eighteenth century two subjects of great 
importance began to occupy the minds of geologists; the one was 
the order or sequence in which the rocks had been laid down in 
the past history of the earth, and the other was the nature of the 
fossils that were found imbedded in them. The two subjects 
were very soon discovered to be closely linked together; indeed, 
erelong it was seen that a close study of the fossils would pro- 
vide the key to the succession of the rocks. There were two 
districts where such a succession could be traced without much 
difficulty, for in these regions the layering of the strata had 
not been nearly so much disturbed in bygone ages. These 
districts were Central France and Eastern England. 

Take, for example, an imaginary section across England 
from Snowdon to the Wash (Fig. 48). In North Wales we 
find the very oldest rocks tossed up into high mountains (P); 
then follows a rather steeply inclined set of beds in which we 
find coal (C), and then a broad flat plain, represented on our 
ordinary maps by the counties of Cheshire and Shropshire 


(NRS). The coal measures appear again when we approach 
the Derbyshire hills, while away eastwards are low-lying 
almost horizontal strata, gently tilted towards the west and 
ending in the plains of Lincoln and Norfolk. It will be seen, 
therefore, that a traveller walking from Yarmouth to Holyhead 
passes over successively older rocks, since the more recent 
(overlying) strata have been worn away to a greater extent 
towards the west. 

But the strata are by no means so simply arranged in other 
parts of the country, and the problem for the geologist is to 
say what is the order of succession where the layers are bent, 
twisted, and overturned, or even largely worn away, so that 
only fragments of them are left, jumbled up with other layers 

c -^^-^^6^^^^=^^==^ 


in great confusion. Let us now see how the fossils may help us. 
Suppose we meet with a bed of rock containing shells that are 
quite unknown to the student of living Mollusca, and another 
bed containing the same forms, but with a sprinkling of new 
ones rather more like those now living. In another layer still 
the old types of shell appear to be dying out, for only a few 
scattered examples can be found, while what we may call the 
second type are abundant. The next layer contains plentiful 
specimens of the second type, but mixed with a third set still 
more closely resembling living forms, but the first type has 
disappeared altogether. Obviously, if we should meet with 
rocks in any other part of the country containing fossils all 
belonging to the first type, we should be justified in saying that 
these rocks were of the same geological age as those in which 
we first identified them, and similarly for the other two kinds 
of strata. In this way, then, we might be able to trace a 
particular bed of rock all over the kingdom, even though these 
fragments might be many miles apart. For example, in the 
section given in Fig. 49, taken across Merionethshire from 


Tremadoc Bay to the vicinity of Bala Lake, the beds marked A 
contain well-marked fossils, and across country miles away 
we meet with another bed of rock, A', in which such fossils also 
occur. We have no hesitation in saying that A and A' are 
outcrops of the same stratum, but that long ages of denudation 
have swept away all the connecting parts from the top of what 
must have been once an immense hump or " anticline/' as the 
geologists call it, represented by the dotted lines. 


There were many at the end of the eighteenth and the 
beginning of the nineteenth centuries who worked at problems 
such as these, tracing the occurrence of similar rocks all over 
the kingdom and mapping them out with great patience and 
skill One of these pioneers was an Oxfordshire man called 
William Smith. He was born in 1769, and, after receiving 
a rather scanty education, became an assistant to a land 
surveyor. His duties took him to every part of the kingdom, 
and being greatly interested in geology, he made abundant notes 
of everything that might have a bearing on the succession of 
strata. It was not long before he became certain in his own 
mind that there was such a succession and that each layer 
contained fossils special to itself, which would enable it to be 
traced wherever it might occur. After collecting quite a mass 
of data, he plotted out his results in the form of a geological 
map, one of the first ever produced for Great Britain . Although 
he published little else, still Smith must be remembered as 
a pioneer in what is now called stratigraphical geology. 

Both before and after Smith's day the workers in the subject 
increased rapidly, and data were accumulating to such an 
extent that it soon became possible to exhibit the whole series 


of rocks in the order of their ages in the form of a table 

(Fig. 50). 

The most ancient, or Primary, rocks were studied by men like 
Sir Roderick Murchison, who devoted himself to a series of 
very ancient deposits on the borders of Wales which he called 
Silurian, after the tribe of the Silures 
who inhabited that region in Roman 
times. The results of his work were 
published in 1838 in a huge volume 
of over 800 pages, along with an atlas 
of plates of fossils and sections show- 
ing the distribution of the beds. 

Murchison was closely connected in 
his researches with another distin- 
guished geologist, Adam Sedgwick, a 
Yorkshireman, who was born in 1785, 
in the Vale of Dent, where his father 
was vicar. When he became professor 
of geology at Cambridge he had only 
a scanty knowledge of his subject, but 
what he lacked he soon acquired by 
an enthusiastic study of the very 
complex geology of the Lake District, where he discovered 
the presence of volcanic rocks wedged in between layers that 
had obviously been laid down in the sea. It is to him that 
we owe much of our knowledge of the rocks lying imme- 
diately above the Silurian, called the Devonian and Old Red 
Sandstone. He also grouped together the layers below the 
Silurian as Cambrian, after the old name for Wales, where they 
were so well developed. 

The tale was completed when Sir A. C. Ramsay described 
rocks of greater age even than the Cambrian, in which no fossils 
of any kind could be found, and which he called Precambrian 
or Azoic i.e., without life. The British Isles, it will be seen, 
provide us with examples of practically every kind of rock 
of which the earth's crust is made, and, taking into account 
their extent, a little over 125,000 square miles, we may say 



o , 





c - 

(O ^ 




k ^ 

^ Eocene 





u"c ^ 


2E N 

O O 



' .Permfan 


Q\ Grit 
5 i Limestone 


iw " 

Devonian & 

O t 

5 P 

Old Red 










Azo'ic Precambrian 

FIG. 50. 


that there is perhaps no area in the world of equal extent that 
can show so great a variety of geological formations. 

Geological Textbooks. The immense increase in our know- 
ledge of the details of all these various strata during the later 
years of the eighteenth and the early years of the nineteenth 
century made it quite impossible for even an educated man 
to follow what had been accomplished, and to appreciate 
what had yet to be done, so that the student, and the general 
public also, hailed with joy the appearance in 1833 of the 
excellent summary of all that was then known on the subject, 
called " The Principles of Geology/' from the pen of Sir Charles 
Lyell. Ramsay said of this book; " We collect the data and 
Lyell teaches us to comprehend the meaning of them/' But 
Lyell did more than merely describe and interpret what others 
had discovered; he pointed out the similarity of the fossils in 
the most recent rocks, known as the Tertiary, with those forms 
at present living, and coined names to indicate the chief stages 
in Tertiary formations Eocene, the dawn of the recent; 
Miocene, the less recent; and Pliocene, the more recent. 

Ramsay also wrote an excellent account of the structure 
of Britain, in which he described the scenery of our islands, 
and showed how that scenery depended on the nature of the 
rocks that went to form the hills, valleys and plains. This 
subject, which appealed most of all to the traveller and the 
tourist, was, in later years, made much of by Sir Archibald 
Geikie, who died in 1924, after holding the chair of geology 
in Edinburgh University as well as the post of Director of the 
Geological Survey. To him we also owe the famous " Text- 
book of Geology " that has been the student's guide for very 
many years. 

The Action of Glaciers AGASSIZ. There is another name 
that stands out prominently in the history of Geology, that of 
one who opened up an entirely new subject, whose existence 
had been hinted at, however, by Hutton in his " Theory of the 
Earth/' This was Louis Agassiz. He was a Swiss by birth, 
but spent much of his life in the United States. Before he 
crossed the Atlantic he explored the Alps, and wrote the story 
of what he had seen, which he published in 1837. He described 


how lie had found " erratics/' or wandering boulders, high up 
on the slopes of the Jura mountains, boulders which were 
composed of minerals that were not found in these mountains, 
but only in the Alps, many miles away. They lay far above 
the level of the glaciers that now filled the Swiss valleys, and, 
near these blocks, the rocks were all polished and scratched, 
just as were the rocks below the glaciers of his native 
country. He concluded that once on a time an immense sheet 
of ice must have spread over the plains between the Alps and 
the Jura, and had even climbed over the crests of the latter 
mountains. A glance at an atlas shows that the long ridge 
of the Jura, bounding the eastern margin of France, is, through 
most of its length, fully fifty miles distant from the Alps. The 
ridge in places rises to a height of over 5,000 feet, and if great 
masses of Alpine rock are found high up the slopes, the only 
means by which they could have been carried there was by ice, 
which must therefore have been several thousand feet in thick- 
ness. This was a sufficiently startling idea, and was at first 
looked at askance by many geologists; but Agassiz piled proof 
upon proof, until the doubters were at last convinced that his 
theory was correct. If so, Agassiz said it could mean only one 
thing viz., that the climate of Europe at that period must 
have been somewhat similar to that of Greenland at the present 
day, where glaciers come right down to the shores of the sea. 

The Great Ice Age. Agassiz next visited the Highlands 
of Scotland, the Lake District and the mountains of Wales, 
and in all of these places he found the same evidence of glacier 
action where there were now no glaciers to be seen. One of 
the chief authorities on what is now always called " The Great 
Ice Age" is James Geikie, who succeeded his brother, Sir 
Archibald, in the Edinburgh chair, and who died a few years 
ago. He wrote an important book on the subject, giving 
all the evidence that had been collected. This unexpected 
phase in the history of the earth appeared just after the 
Pliocene period (Fig. 50), when the lower Quaternary rocks 
were being laid down, and seems to have affected not only 
Europe but the whole of the northern hemisphere, more 
especially Canada and the United States, What the cause of 


changes of climate may be is considered later (p. 358, see also 
p. 485). Geologists are not even yet agreed on the subject, 
but what it is interesting to know is that at that time some 
authorities say about 200,000 years ago Great Britain and 
Ireland were connected with the Continent; there were no 
Straits of Dover, and it was possible to walk dryshod from the 
site of London to that of Boulogne or Brussels. 

When the Ice Age began and the climate became colder 
and colder, the mountains of Britain and Scandinavia were 
covered with vast snow and ice fields, and glaciers streamed 
down the valleys and spread over the plains, until northern 
Britain was covered with an ice sheet which is believed to have 
been in some places 4,000 to 5,000 feet thick. " The whole 

of Northern Europe, Canada 
and the northern part of the 
United States," writes Sir 
Archibald Geikie, "was 
buried under a continuous 
mantle of ice. In Europe 
(Fig. 51) the southern edge 
of the ice sheet must have 
lain to the south of Ireland, 
whence it passed along the 
line of the Bristol Channel 
and thence across the south of England, keeping to the north of 
the valley of the Thames. The whole of the North Sea was 
filled with ice down to a line which ran somewhere between 
the coast of Essex and the present mouths of the Rhine. 
Eventually, and no doubt very gradually, after episodes of in- 
crease and diminution, the ice finally retired towards the north, 
and with it went the Arctic flora and fauna that had peopled 
the plains of Europe, Canada and New England. The existing 
snow fields and glaciers of the Pyrenees, the Alps and Norway 
in Europe, and of the Rocky Mountains in North America, are 
remnants of the great ice sheets of the Glacial Period, while 
the Arctic plants of the mountains, which survive also in scat- 
tered colonies in the lower grounds, are relics of the northern 
vegetation that once covered Europe from Norway to Spain/' 



Unless one has actually seen the great snow fields of 
Labrador, where Grenfell pursued his missionary labours, or 
the mighty glaciers that Norman Collie and his friends explored 
in the Selkirks, it is not easy to realise what Britain looked 
like in those far-off days, when "Old Father Thames/' as 
Rudyard Kipling wrote, told the " Twenty bridges from 
Tower to Kew " how 

" these waters of mine 
Were once a branch of the River Rhine, 
When hundreds of miles to the East I went 
And England was joined to the Continent. 
I remember the bat-winged lizard-birds, 
The Age of Ice and the mammoth herds, 
And the giant tigers that stalked them down 
Through Regent's Park into Carnden Town.' 1 

We have now brought the story of how the structure of 
the earth's crust was unravelled down almost to our own time, 
and we may leave it there and turn to the next great science, 
physics, and try to trace what progress it had made since 
Newton studied the nature of light, and Boyle discovered the 
law of the compressibility of gases. 


Ultra-Violet and Infra-Red Rays. Newton, towards the 
end of the seventeenth century, had discovered (p. 51) that 
white light could be split up into a spectrum of rays of different 
colour red, .orange, yellow, green, blue, indigo and violet 
and found that the red rays were the least bent, or refracted, 
and the violet most. Beyond the red on the one side and the 
violet on the other, no rays of any kind could be detected. More 
than a century later Sir William Herschel endeavoured to 
determine the relative heating power of the different regions 
of the solar spectrum by exposing thermometers in the various 
coloured bands. When he placed a thermometer in the path 
of the violet rays he obtained a slight rise in temperature, but 
beyond the violet there was no response at all. It was quite 


otherwise with the remainder of the spe'ctrum. As the ther- 
mometer was shifted towards the red end the temperature 
steadily rose, but, to his surprise, it went on rising after the red 
colour ceased, so that it appeared that the heat rays of the sun 
were mostly dark in what is now called the infra-red, beyond the 
limits of visibility. Herschel concluded that light rays and heat 
rays were of the same nature, but that while the human eye 
could recognise only the rays from the violet to the red, the ther- 
mometer could detect not only the visible rays but also others 
which were even less refracted than the red ones; and he 
confirmed his discovery by showing that the infra-red heat rays 
could be refracted and reflected by lenses and mirrors in the 
same way as the light rays. 

Soon afterwards a physicist called Ritter discovered that the 
region beyond the violet, where the rays had no such heating 
power, was able to cause a blackening in certain compounds 
of silver, so that there must be rays there also, although neither 
the eye nor the thermometer could detect them. The word 
"spectrum/ 1 meaning an image or appearance, is thus a 
misnomer, since much of it cannot be seen at all. 

Wave-Lengths in the Spectrum HUYGENS, YOUNG. In 
1802 Thomas Young produced a very important treatise on light, 
in which he showed that the real difference between red and violet 
rays lay in their wave-lengths, and that the red rays were very 
nearly twice as long as the violet ones. What this means may 
be understood from the following simple experiment. Obtain 
a length of clothes-line and fasten one end to a clothes-pole 
or a hook in the garden wall. With the free end in the hand, 
stand ten or more feet away, keeping the rope fairly taut. 
Then move the hand up and down, when waves will be seen 
to run along the cord from the hand to the wall. If the hand 
be moved rapidly the waves will be short and the succession 
or " frequency " of the waves rapid, if slowly the waves will 
be longer and less frequent. Although we may use this analogy, 
heat or light waves are very different, being vibrations, not 
of matter, but of the hypothetical ether (p. 54) and are all 
exceedingly short. For instance, the length of a yellow wave 
is only about ^ w inch. 


Since the time of Herschel and Young we have learnt to recog- 
nise all sorts of ether waves besides those we can see and feel, 
not only beyond the heat waves outside the visible red, but also 
beyond the silver-blackening waves outside the violet. The full 
spectrum is thus vastly longer than Herschel or Young or any of 

the other workers ftartimarfltMs fl [ 0-000,000,000,1 cm 

on the subject a 
century ago had 
imagined. A 
glance at Fig. 52 
shows what a very 
small part (e) of 
the entire spec- (d) 
trum is visible to Visible(e) 
our eyes. All the 
rest of it can be 
distinguished only 
with the aid of 
very delicate 





Infra red 


0-OOQ,OOO r OOt,C.m. 

0-000,000,1 C.m. 
0-000,001. C.HK 

*\ 0-000,02 c.m. 
0-000, 038, C.m. 

0-000, 078, C.m, [-Solar spectrum 

J 0-005 c.m. 

0-035, c.m. 


Radio waves 1 

(i) i 

1 c.m. 

5-000 c.m or 50m. 

struments speci- 
ally designed for 
the purpose. In 
the figure, how- 
ever, may be noted 
two kinds of rays : 
first, the X rays 

beyond the violet VE==I 2-000,000 c.m.or 20,000 m. 

end of the Visible p IGt $2. THE SPECTRUM, SHOWING THE EXTENT 

are now made so THE REMAINDER - ( AFTER HAL *-> 
much use of in surgery, and far beyond the infra-red, the radio- 
waves. The figures in Fig. 52 show the wave-lengths in 
centimetres (cm.) or metres (m.). Recently the spectrum has 
been extended beyond the 7-rays (Gamma) by the discovery by 
Millikan that the so-called " Cosmic Rays " are of excessively 
short-wave-length. But the way in which all these new rays 
were discovered must be left over for the present. 

Before leaving the subject of light in so far as it was under- 


stood at the end of the seventeenth century, we must again 
refer to two men whose names we have already mentioned, 
Christian Huygens and Thomas Young. 

Huygens was born in 1629 and died in 1695, so that he was 
a contemporary of Newton. Although intended for the legal 
profession, he soon showed himself as a very competent mathe- 
matician and physicist. Newton, in his efforts to make a 
satisfactory refracting telescope, was foiled by chromatic 
aberration (p. 52), and realising that the difficulties he met 
with were due not to defects in the lenses but to the nature 
of light itself, gave the matter up, and invented the reflecting 
telescope instead. Further, Galileo, with his very primitive 
instrument, failed to solve the problem of the curious triple 
condition of Saturn (p. 31). Huygens set himself the task of 
elucidating both these puzzles. 

First of all, he succeeded in finding a new way of grinding 
and polishing lenses, and, with a much improved instrument, 
he was able to show that Saturn was surrounded by a ring 
set at an angle to the ecliptic, and which went through phases 
in a period of years. He was so successful with his work 
on lenses that he made some that were almost perfectly 
achromatic. He mounted these on lofty poles, giving a focus 
of 100 to 200 feet, the sort of instrument that Bradley used 
in his researches on nutation (p. 74). Huygens's greatest dis- 
covery had to do with light, and we have seen how he replaced 
Newton's " corpuscular theory " with the " undulatory theory" 
that is always associated with his name (p. 54). 

Another feat performed by Huygens was the interpretation 
of the refraction of rays of light when they passed from a less 
dense into a more dense medium, such as from air into glass, 
in terms of his undulatory theory, and this is the explanation 
he gave. When a wave of light strikes a plate of glass at right 
angles to its surface, it moves more slowly through the glass, 
but passes out on the other side unchanged and in the same 
direction, both sides of the beam, so to speak, being equally 
retarded by the glass plate. But if it strikes the plate at an 
angle, one side of the wave-front will strike the plate before the 
other and be retarded, while the other, moving at the original 


rate, will swing round and thus the direction of the whole beam 
will be altered. Similarly, after passing through the glass, one 
side of the beam will come out first, and, being now in a less 
dense medium, will move more rapidly, while the other side, 
which has not yet come out of the glass, is still moving more 
slowly, and so the beam suffers another swing, bringing the 
whole back to the original direction but on another parallel. 
When the ray is passing through a lens the same thing takes 
place, but the new direction will not be the same as the original 
one. In a biconvex lens the rays will be bent twice in the 
same direction, and consequently will converge or come to 
a focus, while in a biconcave lens they will be bent twice in 
opposite directions and so be dispersed. 

About this time a Danish doctor, called Bartolinus, obtained 
from Iceland a mineral which he called <( Iceland Spar/' a 
substance known to chemists as calcite. The crystals of this 
mineral are in the form of rhombohedra (Fig. 53), and on examin- 
ing one of them he noticed that it behaved in 
a very peculiar manner with regard to light, 
for he found that when he looked at a small 
object through the crystal it appeared double. 
Huygens came across Bartolinus's account 
of this curious phenomenon and proceeded 
to investigate it, and gave his results in 
his "Treatise on Light/' published in 1690. 
In all other transparent bodies known to 
him there is one simple refraction, but in 
Iceland Spar there are two. In ordinary cases, when a ray 
falls perpendicularly on a transparent surface, it passes through 
the body without any refraction, but if it falls obliquely it 
is refracted. In Iceland Spar, however, the perpendicular 
rays are refracted and the oblique rays pass straight through 
It appeared to Huygens, therefore, that the crystal was more 
" elastic " in one direction than in the other. He further 
discovered that if the two rays were made to pass through a 
second crystal they remained separate and did not change 
their direction. He next found that if he turned the second 
crystal slightly round on its axis each of the two rays was 



again split into two, so that he got four rays of varying 
brightness. When the second crystal was at right angles to 
the first i.e., had been turned round 90 (Fig. 53) the two 
rays reappeared, but had changed their characters, for the 
one that had behaved normally now behaved abnormally and 
vice versa. Huygens confessed himself unable to explain this 
curious result, and it remained unexplained for over 100 years, 
until further discoveries led to the study of a new chapter 
in optics called "polarised light." That we shall come to in 
due course. 

We are now approaching the nineteenth century, during 
which, perhaps, more great discoveries were made and more 
remarkable men lived than in any other century of our era. 
Our chief difficulty will be to decide what and whom to include 
and what and whom to omit, for to treat of all would be 
impossible. But there is another difficulty that faces us, and 
that is, how far may we follow a purely scientific discovery 
into its practical applications ? Take two examples to illus- 
trate this point. A discovery of great importance in pure 
physics was made by Professor Joseph Black of Edinburgh 
University, towards the end of the eighteenth century, and 
had it not been for that discovery James Watt could never 
have been able to improve (for he did not "invent") the 
steam engine in such a way as to make it what the Americans 
would call " a paying concern/' Then, again, another great 
man, Michael Faraday, induced an electric current in a coil 
of wire by thrusting into it a powerful magnet, and this coil 
and magnet was the forerunner of the modern dynamo. The 
practical applications of what are apparently very simple 
scientific discoveries are so numerous that, were we to discuss 
a tithe of them, this little book would rapidly expand into a 
library* We must therefore content ourselves with mentioning 
only a very few. 

Perhaps the best way of gaining a knowledge of the ad- 
vances made in physics during the later years of the eighteenth 
and the earlier years of the nineteenth centuries is to consider 
the chief departments of the science separately sound, light, 
heat and electricity and as it does not much matter which of 
these we take first, we may begin with sound. 



What is sound ? If a tuning-fork be pinched and the 
prongs released smartly, one hears a musical note, and if the 
tips of the prongs be watched closely, they will be seen to be 
vibrating. The vibrations set up waves in the air which 
beat against the drum of the ear and give us the mental 
impression of a musical note, let us say C, on the pianoforte. 
If we pinch the prongs gently the note sounded may be so 
feeble that we must put the fork close to our ear in order to 
hear it; if we pinch them strongly, or, better still, place the 
tip of the handle on the lid of an empty box, the sound is much 
louder but of the same "pitch," as it is called. The pitch is 
determined by the number of vibrations of the prongs per 
second and the number is the same whether the sound be 
faint or loud. The loudness depends on the range of the air- 
waves from the fork to the ear, known as the " amplitude "; 
the greater the amplitude the louder the sound, 
but the pitch remains the same. Some tuning- 
forks are provided with sliding bars (Fig. 54) 
which can be set to marks on the prongs corre- 
sponding to the different notes of the scale. If 
the bars be moved from C to D, let us say, the 
pitch will be a full tone higher; by so doing we 
have altered the " frequency" of the vibrations, 
and the higher the frequency the higher the pitch. 

The pianoforte keyboard is divided into octaves, 
or spans of eight notes, and between any two of 
these notes there is a musical "interval/' which 
depends on the frequency of the vibrations of 
the two successive wires struck when the corre- 
sponding keys are touched. This difference in frequency is 
called the "ratio/ 1 Thus if one note has a frequency of 
1,000 vibrations per second, and another a frequency of 500, the 
ratio is f , meaning that the first wire is vibrating twice as fast 
as the second (octave). So long as the ratio is the same, it does 
not matter where one begins to play a scale. A singer may be 

FIG. 54. 


accompanied on any " key/' or, in other words, the accom- 
paniment may be "transposed'* to suit the range of the 
singer's voice. As many players are not sufficiently expert 
pianists to transpose an accompaniment at sight, songs are 
often printed in two or more keys to suit the performer. In 
the pianoforte we have a succession of musical scales, and if we 
call the ratio of the first note of the octave -, the next will be 
f, then f , |, f , |, V 6 an( i f> so ^hai *^ e e ighth note above that 
from which we started has twice the frequency of the note 
an octave below. In actual practice half tones are introduced 
between C and D, D and E, F and G, G and A, A and B, 
represented by the black keys, so that the scale on the piano 
really includes thirteen notes. 

When two or more notes are sounded at the same time 
they may be in " accord " or in " discord " to our ear, and 
when a pianoforte is " out of tune," it means that the fre- 
quencies are not in their proper ratios. Again, when a note 
is struck on an instrument, it is very rarely pure; there are 
almost always what are called " overtones," or vibrations of 
greater frequency (octaves and higher), and these overtones, 
so long as they are in harmony with the base-note, give the 
special " timbre " or quality to the instrument. 

Sounds of irregular frequency, like, say, the tapping of a 
hammer, do not impress us as musical notes at all, but merely as 
noise. The lower limit of a distinctly musical note is about thirty 
vibrations per second and the highest about 35,000; anything 
higher than that is, as a rule, quite inaudible. Few people can 
distinguish notes of above 25,000 vibrations per second, and 
as a person's age increases the ear drums usually become less 
sensitive, and the power ,of appreciating sounds, and especially 
high ones, becomes less and less. 

In early days several investigators paid attention to this 
subject, and prominent among them was Joseph Sauveur, 
who was born in 1653 and died in 1716. Most of his experi- 
ments were made on air waves within organ pipes, and his 
results are all the more remarkable since his speech and hearing 
had been very imperfect from early boyhood. The conclusions 
he came to were, however, fully confirmed by a great physicist 


of the nineteenth century, called Helmholtz, who was also, 1 
distinguished by other discoveries in the science of physics. 

The subject of " acoustics/' as it is sometimes called, was 
studied afresh by Ernst Chladni, who was born in Saxony in 
1756. Chladni, like so many other young scientists, was 
trained for a profession law for which he had no taste, but 
which was forced on him by his parents. After his father's 
death he began to devote himself to music, of which he was 
passionately fond, and, on finding that 
so little had been written on the physi- 
cal aspect of sound, he determined to 
discover if possible some of the laws of 
harmony. He was acquainted with the T j 
curious figures that appear on sheets of 
glass when sprinkled over with powders 
of various kinds and exposed to the action 
of magnets, and this gave him the idea 
of trying to produce similar effects by 
striking or rubbing plates of metal treated 
in the same way. He fastened a thin sheet FIG. 55- A, CHLADNI'S 
of brass, B, in a vice, V (Fig. _ 55 , A) and % .*' ** * 
rubbed the edge of the sheet with a violin 
bow. The result was a musical note due to the vibration of 
the plate, and this was accompanied by the arrangement of 
the grains of powder in the form of a star or other pattern 
(B), the number of rays being dependent on the number of 
points where the vibrations of the plate were " damped " or 
stopped by placing the fingers on the margin of the disc. He 
obtained some very remarkable results in this way, according 
to the shape of the plate, the place where the violin bow was 
applied, and the number of points where the vibrations were 
damped. " Chladnf s figures " formed the subject of mathe- 
matical investigations during the nineteenth century, but into 
a discussion of these we need not enter. 



At the end of the seventeenth century, it will be remembered, 
there were two theories as to the transmission of light, both of 
which had their supporters viz., Newton's " corpuscular 
theory" and Huygens's "undulatory theory." Newton's 
great name and reputation led most people to adopt his view 
of the phenomenon, but, on the other hand, Huygens's theory, 
although it postulated the existence of a universal medium 
pervading all space, an " ether " which could not be seen, felt, 
weighed or in any other way identified, seemed to have much 
in its favour. 

Thomas Young, to whom we have already referred, was a 
most versatile person. He was not only an excellent linguist, 
for it is said he could speak seven languages, but he was also an 
authority on hieroglyphics, a subject which engaged the atten- 
tion of Newton in the later years of his life. In his own pro- 
fession of medicine he showed how the curvature of the 

crystalline lens of the eye 

^.?... ^ .~-~^ was unconsciously altered 

/* -' '~v.~r" >x ^ \ to accommodate it for 

^;;^:::'^ -*'* " ' '" * ^v~v.-.-J\. vision of objects at vary- 

R Y G B ! v ing distances. In relation 

FIG . ^.-PRIMARY COLOURS ACCORDING co i our he held that there 

TO YOUNG. , . 

were only three primaries, 

red, green and violet (Fig. 56), and that all other colours were 
due to the mingling of these in different proportions. He also 
asserted that there were three sets of nerve endings in the 
retina which received these sensations and conveyed them by 
the optic nerve to the brain. This view received considerable 
support long afterwards when the pathological condition known 
as colour blindness came to be investigated. 

The Doctrine of Interference YOUNG. It was only to be ex- 
pected that Young should be led to study the subject of optics in 
general, and to attempt to settle the question whether Newton's 
views or those of Huygens were correct. His contribution 
to the discussion was his exposition of the " doctrine of inter- 
ference/* A simple illustration will make this subject clear. 


Consider two sets of water-waves meeting each, other and pass- 
ing into a narrow canal (Fig. 57). Two possibilities then arise. 
When the two sets of waves reach the mouth of the canal the 
crests of one set may coincide with the crests of the other (B), 
and the hollows between the successive crests may also coincide; 
on the other hand, the crests of one set of waves may coincide 
with the hollows of the other set (A). In the former case the 
crests unite, and the wave 
passing into and along the 
canal will be larger than 
either of the two waves 
that combine to make it. 
In the latter case the crests 
of one set of waves will 
fill up or neutralize the 
hollows in the other set, 
and an approximately level 
and calm water ensues 
along the canal. 


Working on such a basis, Young proceeded to study waves 
of light, on the assumption that the undulatory theory was 
the correct one. If a ray of light of uniform wave-length be 
allowed to enter a dark room through two minute apertures 
placed close together, the two rays will interfere with each 
other, and if a screen be placed some little distance from the 
apertures, the result will be a number of bright bands separated 
by dark ones. The central bright band, all parts of which are 
at the same distance from the aperture, is formed by the union 
of two waves of light whose crests and hollows are combined, 
crest on crest and hollow on hollow, as in B in our example of 
water waves (Fig. 57). The same is true of the bright bands 
on either side of the central one. What of the black bands 
between them ? In the middle of these the light waves meet 
each other, crest to hollow and hollow to crest; they neutralise 
each other, and the result is darkness. When we use ordinary 
sunlight for our experiment, we find it impossible to obtain 
absolutely black intermediate bands, because the sunlight is 
made up of rays of different wave lengths. Hence if the red 


rays interfere with each other the blue ones may not; on the 
contrary, they may even intensify each other, and that is 
why we always find pale-coloured bands when the light is not 
pure or of uniform wave-length (monochromatic), 

Polarised Light. The proble'm of the strange behaviour of 
crystals of Iceland Spar in relation to light (p. 109) was reopened 
by an accidental discovery made by the French engineer, 
Malus. On one occasion he was examining, through a crystal 
of Iceland Spar, light reflected from a window opposite his 
lodging, and noticed that, when he held the crystal in a certain 
way, he saw not two images but one. On changing the position 
of the crystal the double image appeared, but one of the Images 
was brighter than the other; on turning the crystal a little 
further, he got once more only a single image. He next dis- 
covered that it was only when the reflected light came at a 
certain angle that the crystal behaved in this remarkable 
manner, and that the angle differed according to the nature of 
the substance from which the light was reflected. Light that 
behaved in this way he called " polarised light/' 

The subject was at once taken up by Young, and also by a 
Frenchman named Fresnel who was born in 1788 and died 
in 1827. He spent his short life in retirement in Normandy, 
for, being a royalist, he was not in favour with Napoleon and his 
government. Fresnel was an intimate friend of Young, and 
when the problem of polarisation was reopened by Malus, both 
these investigators made efforts to explain the phenomenon. 
The explanation they arrived at was, very roughly, as follows; 

Waves of ether like water-waves travel at right angles to the 
plane of oscillation. Take a compass card and join opposite 
points, N-S, W-E, and any other opposites in between, and 
then pierce the centre of the card with a needle which will 
indicate the path of the ray, the plane of the card being kept 
vertical to the needle. The waves of light will always be in the 
plane of the compass card from any one point to the opposite 
one. Crystals, however, have the peculiarity of permitting 
waves to pass along two pathways only, and these pathways 
are at right angles to each other i.e., if the one path be N-S, the 
other is W-E. Hence a crystal splits a beam of light into two 


beams which travel at different rates, and are therefore re- 
fracted unequally. Each beam is called a beam of " plane 
polarised light." Different crystals behave in different ways 
in relation to the rays of 
light. Thus tourmalin, a 
very beautiful mineral, 
first obtained from Tour- 
mali in Ceylon, absorbs 
one of the two rays, and 
hence the light passing 
through it is " plane polar- 
ised." If two crystals of 
tourmalin be superposed, 
A (Fig. 58), they allow only one beam to pass through, but if 
one of the crystals be turned through an angle of 90 so that 
they cross each other at right angles (C), the ray that passed 
through the first crystal is "interfered" with, and blocked by, 
the second crystal. It is on this principle that the instrument 
known as the polariscope is constructed. 



In the early years of the eighteenth century, and, indeed, 
for long afterwards, heat was believed to be a substance which, 
later, received the name of " caloric." It was regarded as " an 
extremely refined fluid that was able to force its way between 
the ultimate particles of the densest bodies." This conception 
of the nature of heat was almost universally accepted until the 
very end of the century. It seems strange to us that scientific 
men even in those early days did not realise that since a hot 
body was no heavier than the same body when cold, heat could 
have no weight, and that since weight was a property of matter, 
heat could not be matter. 

In the year 1728 there was born, at Bordeaux, a boy called 
Joseph Black, whose father was a Scotchman carrying on the 
business of a wine merchant there, and also in Belfast. Joseph 
was educated first at Belfast and afterwards at Glasgow 
University, where, in time to come, he became professor of 


medicine and chemistry, ultimately removing to Edinburgh 
to hold a corresponding position there. It was in Edinburgh 
that Black made the acquaintance of Hutton, the author of 
the "Theory of the Earth/' What Black did for chemistry 
will be discussed later on; here we are concerned only with his 
discoveries in physics. 

Heat and Temperature BLACK. In order to understand 
what these discoveries really were, it may be as well to glance 
at certain general facts, of which we have all had experience. 

What exactly do we mean when we say that such and 
such a thing is hot or cold ? Our own bodies have a certain 
temperature which never changes appreciably; it is always 
about 98-5 of the Fahrenheit scale. In the warmest regions 
of the earth, where the outside temperature may be far above 
ioo u F., the temperature of the body does not rise over i from 
the normal, and in such terrible districts as Northern Siberia, 
where a temperature of -88 F. has been recorded, still the 
body temperature seldom sinks below 98 F. Should the 
doctor's thermometer reach 103 F., we are regarded as 
" feverish "; our condition becomes grave if it reaches 105 F., 
and anything above that, if long continued, is generally fatal 
Our body, therefore, gives us a standard to go by, and when 
we say it is " very hot " on a blazing July day, or " very cold " 
in mid-winter, we are comparing the climate with our own 
almost constant temperature. * 

But further consideration of the facts leads us to the belief 
that the human body is, after aU, not a very reliable judge. A 
very well known and often quoted experiment soon convinces 
us that heat and cold are not quite so easily defined as might 
at first sight be imagined. Obtain three vessels and fill one 
with very hot water, the sefeond with ice-cold water, and the 
third with tepid water. Place the right hand into the hot water 
and the left into the cold, and after a few moments, transfer 
both hands to the tepid water. The right hand will tell us 
that the third vessel contains cold water and the left hand 
will say it is hot ! Which hand is to be believed, for obviously 
the water in the third tumbler cannot be hot and cold at the 
same time ? 


Take another simple experience. Put half a pint of cold 
water into a chemical beaker a tumbler made of fire-resisting 
glass and place it over a spirit lamp for, say, a quarter of an 
hour. The water by that time will be hot, if not boiling. Put 
ten half-pints of cold water in a copper kettle, and place that 
over the spirit lamp for the same time. At the end of a quarter 
of an hour the water will be barely warm. Yet the beaker with 
its half-pint and the kettle with its ten half-pints have both 
received the same amount of heat. Heat and temperature are 
thus two different things; two bodies may contain the same 
amount of heat and yet be at very different temperatures. 

Again, suppose we place a block of ice in a pan on a hot 
stove; of course, as we might expect, the ice will begin to melt. 
Stir the ice about in the pan so as to mix the resulting water 
thoroughly, and it will be found that, so long as any ice is left 
unmelted, the temperature of the water remains that of ice- 
viz., o C. or 32 F. What has become of the heat ? Keep on 
heating the pan; now, when the ice has disappeared entirely, the 
water will get hotter and hotter until the thermometer tells us 
that the temperature is 100 C. or 212 F. So long as the air 
pressure remains constant, and no matter how much we may 
raise the temperature of the stove or how long we may continue 
the heating, the temperature of the water never rises above the 
boiling-point mark. Again, what has become of the heat ? 

Black's Theory of Latent Heat. Keeping these simple facts 
in mind, let us turn to Black's enquiries into the nature of heat, 
the " subtle fluid/' as it was called in his day. Black's ex- 
periment was very like that we have just made. He filled two 
flasks, one with ice-cold water and the other with an equal 
weight of actual ice, and left them in a room whose temperature 

was kept at 8*5 C. or 4 7'3 F. I* half an hour the water ' ^ 3ch 
at the start had registered o C., had risen to 4 C. (It is 
easy to turn the centigrade scale into the Fahrenheit by re- 
membering the following rule: double the centigrade figure, 
subtract a tenth and add 32. Thus 8-5x2-17; I7~i7== 
IV V 1^+32=47*3.) The ice in the other flask was melting, 
but r'tiJ ? temperature of the mixture of ice and water was 
still o C. Ten and a hall hours later all the ice had melted, 


and the resulting water had risen from o C. to 4 C., a tempera- 
ture that the other flask had reached ten hours previously. 
During all that time the flask with ice in it had been absorbing 
heat, and if half an hour was sufficient to raise the temperature 
of the ice-cold water 4 C., then in the twenty-one half-hours 
the other flask must have absorbed heat equivalent to raising its 
temperature 21x4 or 84, while, in reality, it had raised it 
only 4 C. The obvious conclusion was that all the heat 
equivalent to 80 had been spent in turning the ice into water 
without raising its temperature at all. 

In another experiment Black discovered that if he mixed a 
pound of ice-cold water with a pound of water having a tempera- 
ture of 79 C., the temperature of the resulting mixture was 39-5, 
but that when he melted a pound of ice by means of a pound of 
water at 79 C. , the temperature of the water when all the ice had 
melted was o C. The whole of the heat represented by the 
79 of the hot water had thus become hidden or " latent " in 
the 2 pounds of water, and there was nothing to show for it. 
It was by experiments such as these that Black was able to 
formulate the two important conceptions, which we are familiar 
with as specific heat and latent heat. 

COUNT RUMFORD. The next step forward was taken by 
Benjamin Thompson, better known as Count Rumford. This 
man had a rather adventurous history, and, as this is rarely 
given in books on physics, we may, perhaps, spend a moment 
or two on it. 

He was born in the State of Massachusetts in 1753, and 
began life as a schoolmaster; but, holding royalist views, he 
had to leave America, and became an official of the Colonial 
Office in London. He had always been a keen student of the 
experimental sciences, and for his work on explosives he was 
elected a Fellow of the Royal Society. After a short period of 
soldiering in America, he returned once more to England and 
was knighted for his military exploits. On the conclusion of 
the war he migrated to Bavaria, where he entered the service 
of the Elector, and soon rose to high rank. He proved himself 
to be a man of prodigious energy and great ingenuity. He 
reorganised the Bavarian army and established military schools 


and gun factories, and used his knowledge of engineering in 
draining marshes near the large cities. Seeing that Bavaria 
swarmed with beggars, he introduced a poor law system, based 
on the principle " work for the state and the state will house, 
clothe and feed you, but not unless/' He also took in hand 
the training of the common people in cookery, invented various 
kinds of nourishing and palatable dishes, and all sorts of cooking 
apparatus to render the manner of living more economical. 
He also turned his attention to agriculture, encouraging the 
cultivation of the potato as an article of food, and suggesting 
methods of improving the breeds of horses and cattle. All 
these and other highly important labours brought him honours 
both public and academic, and amongst other distinctions he 
was created a Count. He then took the name of " Rumford/' 
after the original capital of the State of Nwjlampshire. 

Foundation of the Royal Institution. During a visit to 
England he founded, at his own expense, what we know as the 
Royal Institution, where lectures and other courses of in- 
struction might be given in the words of its prospectus 
" for diffusing knowledge and facilitating the introduction of 
useful mechanical inventions and improvements, and for 
teaching the application of science to the common purposes of 
life/' As this institution became the scene of the epoch-making 
discoveries of men like Davy and Faraday, it may be realised 
what Rumford accomplished for the benefit of science and of 
the nation. To this day our most distinguished men of science 
are only too proud to be elected to professorships in the Royal 

Having retired from the service of the Elector of Bavaria, 
he married the widow of the great chemist Lavoisier and 
settled near Paris, where he died in 1814, 

Heat a Mode of Motion. It was during his work in con- 
nection with his gun-factories that he made his most important 
discovery on the nature of heat. One day, while watching the 
boring of cannon in his foundry, Rumford noticed that the metal 
shavings and the gun itself were far too hot to touch, and very 
naturally argued that heat could not be a substance, however 
" subtle/ 1 if it could originate merely by rubbing two bodies 


together. Even when he caused the guns to be bored under 
water heat was produced, for the water in which the boring 
took place became quite hot. He then carried out some ex- 
periments by causing a heavy steel drill to revolve rapidly in 
a hollowed-out block of brass, after filling the cavity with 
water. After a couple of hours the water was boiling, the heat 
being produced by the friction of the steel against the brass. 
His conclusion was that heat is not a substance but a " kind of 

Almost immediately afterwards the English chemist, 
Humphry Davy, who, at Rumford's invitation, became pro- 
fessor at the Royal Institution, obtained sufficient heat to melt 
two blocks of ice by simply rubbing them together. The heat 
obviously could not have come from the ice, seeing that ice 
could not be above o C, (freezing-point), nor could it have 
come from the air, for the experiment succeeded equally well 
when it was performed in the vacuum of an air-pump. So 
heat, as Davy expressed it, is "a peculiar motion, probably a 
vibration of corpuscles of bodies, tending to separate them/' 
The "old name "caloric" thereafter disappeared from science 
as a name for heat, although we still use a word like it 
" calorie "to signify a unit of heat-quantity. 

The change of matter from one state into another as the 
result of heating it, as, for instance, the transformation of 
ice into water or water into steam, simply means that the 
particles, when their temperature is raised, jostle each other 
more and more vigorously, until they escape from each others 
attractive influence, especially in the condition of steam or 
water vapour, while conversely they are more and more 
crowded, or move less and less actively when they assume 
the solid state. There must come a time, therefore, when the 
jostling ceases altogether, and that temperature, which has 
been very nearly reached artificially, is called the " absolute 
zero/' for if there be no motion among the particles there can 
be no heat. This absolute zero has been estimated at 273 C. 
On the other hand, we cannot fix any maximum temperature 
limit, though there are some very high temperatures that have 
been ascertained that may be of interest to mention. 


When we speak of a poker being " red hot," its temperature 
is about 500 C., the temperature at which coal burns. When 
the poker is at a " white heat/' it is over 1,000 C. The electric 
carbon arc gives a temperature of 3,500 C., and the surface 
of the sun is estimated at 6,000 C. Astronomers, or rather 
astro-physicists, tell us that the temperature of the centre of 
the sun is something like 40,000,000 C. ! At the other end of 
the scale, the freezing-point of mercury is 39 C., or, on the 
Fahrenheit scale, over 70 of frost, and the freezing or solidify- 
ing point of the gas hydrogen is - 258 C. 

We said at the beginning of this section that we might 
select for consideration one or two of the best-known inventions 
that arose out of the discovery of natural laws, and that of 
latent heat, due to Joseph Black, gives us an opportunity of 
redeeming our promise, for on this law rests the principle of, 
the steam engine. Ask a friend who it was that invented the 
steam engine, and the chances are that, in nine cases out of ten, 
the answer will be James Watt, and he will probably repeat 
the old tale about Watt having got the idea from watching the 
steam escaping in puffs from the lid of a kettle boiling on the 
hob of his mother's kitchen fireplace. That story has, probably, 
as much truth (or as little) in it as there is in that of King 
Alfred's burnt cakes, or in the tale of Newton's discovery of the 
law of universal gravitation by meditating on the faU of an 
apple in the orchard at Woolsthorpe. 

JAMES WATT. James Watt was born at Greenock on the 
Clyde in 1736, the son of a small tradesman in poor circum- 
stances, who had lost all his money in some speculation. James 
elected to become an instrument maker, and went up to London 
to serve an apprenticeship in that trade. On his return to 
Glasgow the local trades union would not allow him to open a 
shop, but the university authorities, hearing of his skill in 
mechanics, allotted him a workshop within their own boun- 
daries, where Watt undertook repairs to apparatus used in the 
laboratories, and made himself generally useful to the various 
professors who required his assistance. 

Newcomen's Engine. One day the professor of physics 
brought him a model engine that required mending, and it was 


while engaged on this piece of apparatus that he began to study 
the theory of steam engines in general. During the time that 
Watt was mechanic to Glasgow University, Joseph Black was 
professor of chemistry there, and he often wandered into Watt's 
workshop to talk over scientific problems. The two young 
men for they were almost of an age became fast friends. 
Black was at that time engaged on his researches on latent 
heat, and to him Watt propounded his views on the chief 

defects he thought he had dis- 
covered in the model he was 
repairing. The model was that 
of an engine for pumping water 
out of mines, invented by New- 
comen, who was a blacksmith 
in Dartmouth in the early years 
of the eighteenth century. It 
is essential to master the main 
points in its construction if we 
are to grasp the nature of the 
improvements Watt made on 
it, but it is unnecessary to go 
into any detail either of New- 
comen's engine or of Watt's 
modification of it. The figures 
are therefore just sufficiently 
p detailed to illustrate the prin- 
ciples on which the two machines 
were built. 

FIG. 59.-NEwcoMEN's ENGINE. In Newcomen's engine (Fig. 

59) the cylinder, G, stands 

under one end of a beam, M, pivoted on a rigid support, L. 
The cylinder is open at the top; steam from the boiler, A, 
enters the base of the cylinder by a tube, B, on the course of 
which there is a tap, C. When the steam enters the cylinder it 
first drives the air out by the outlet, J, which is provided with 
a valve opening outwards only. The piston and rod, H, I, 
rise, and the beam swings into the position seen in the figure, 
being pulled up by the counterpoise, 0, at the end of the rod, 


N, which in turn passes to the pump, P, in the mine below. 
The tap, C, is now closed, the cylinder at this moment being 
full of steam, but at low pressure. The cylinder is also con- 
nected with a tank of cold water, F, by way of the tube, D, 
which has on its course the tap, E. When E is opened cold 
water flows into the cylinder, causing the steam to condense 
into water and therefore tending to produce a vacuum in the 
cylinder. The atmosphere now presses the piston down to 
the position shown by the dotted lines, thus pulling up the 
rod, N, and its continuation, P, in connection with the pump. 
When the piston reaches the bottom of the cylinder the tap, 
E, is shut and C opened, and the whole performance is re- 
peated. Of course, the alternate opening and closing of the 
taps, C, and E, is carried out automatically by connecting 
them with the swinging beam. 

The apparatus is obviously an exceedingly clumsy one, 
calling for improvements in many ways, but there was one 
item in its construction that Watt rightly regarded as a fatal 
flaw i.e., the great waste of heat and consequently of fuel. 
At every stroke of the piston the entry of the cold water from 
the tank, F, cooled down the cylinder and piston, so that these 
had to be reheated every time the piston rose, using up a large 
amount of heat that might be better employed. 

Watt's Steam Engine. Watt then consulted his friend 
Black and learnt of his doctrine of latent heat, and realised that 
steam contained heat stored up in it, or latent, which had to be 
extracted from it before it once more became water. This, in 
Newcomen's engine, was done inside the cylinder, so cooling 
it, and one evening, it is said, while walking in Glasgow Green, 
tfce idea occurred to him: " If I can condense the steam else- 
where, the cylinder will remain hot, and all this fuel will be 
saved/' Fig. 60 shows how he successfully solved the problem. 

C is the cylinder, now closed at the top, in which the 
piston, K, moves up and down. The cylinder is pierced at 
either end by two tubes in pairs, E, F and G, H. E and 
H are connected to a tube from the boiler, M (not shown), 
and F and G to another tube which connects with the con- 
denser, A. The condenser has an outlet pipe, L, and is sur- 



rounded by another cylinder, B, which has an inflow pipe, I, 
from a tank of cold water (not shown), and another outflow 
pipe, J. The pipes, E, F, G, H, have valves so arranged that 
when E and G are open, as in the figure, F and H are closed, 
and vice vena. The piston-rod, D, may be connected with a 
pump, a fly-wheel, or the driving wheels of a locomotive, or, 
indeed, with any machine in which it is desired to produce 

" motion against resistance," or, 
in other words, to perform work, 
so that we need not represent 
beams or other apparatus beyond 
letter D. 

How does the engine work ? 
Steam comes along the tube M, 
from a boiler, when, let us say, 
the valves E and G are open and 
F and H are closed, and, being 
under pressure, forces the piston 
down to the bottom of the cylin- 
der. Then automatically, by 
means of connecting-rods, not 
shown in the figure, the valves 
F and H are opened and E and 
G are closed. The steam now 
enters by H, below the piston, 
and drives it up, the old steam escaping by the valve, F. From 
F it passes to the condenser, A, enclosed in the jacket cylinder, 
B, filled with cold water from the tube, I, leading from the cold- 
water tank. In A the steam is condensed by the cold jacket 
and escapes as water by the outlet, L. Meanwhile the water in 
the jacket, continuously warmed by the steam in the condenser, 
flows out by the tube, J, and is continually renewed from the 
tank, ready to cool the flow of steam driven intermittently into 
the condenser. 

It will be seen that Watt's invention was an apparatus for 
condensing the steam apart from the working cylinder, so that 
the latter always remained at or about steam temperature. As 
a consequence, all the heat that in Newconaen's engine went to 



warm the cylinder after the condensation of the steam within 
it was now available for doing mechanical work. Another 
improvement was that steam was made to push the piston 
down as well as up, helped by the vacuum in A, and air pressure 
avoided. Newcomen's engine might be described as a half- 
steam, half-air pressure engine, and Watt's as a whole steam- 
pressure engine. 

Conservation and Dissipation of Energy. A new problem 
had now to be faced by the physicists which was the outcome of 
the work done by Rumford and Black. Their experiments 
had shown that mere mechanical labour could be turned into 
heat. Work means expenditure of energy or, conversely, energy 
is the power to do work. Anyone who works extremely hard 
is said to be " very energetic/' and it does not matter what 
the nature of the work is. It may be actual visible labour, 
such as running up a hill or pulling an oar, or invisible work, 
such as composing an essay or solving a problem in geometry. 
Even when one is asleep the heart is beating, the alimentary 
canal is digesting, the brain is receiving and sending messages 
from and to muscles and glands. All this is work, visible and in- 
visible, and its performance requires the expenditure of energy. 

Any kind of body possesses energy if, by its means, work 
may be done. The energy may be hidden in it, ready to be 
called forth under certain conditions. Thus one's arm at 
rest contains energy which may be employed in lifting a weight 
or pulling an oar. The coiled-up spring of a watch contains 
energy which, when released, causes the wheels to revolve. 
A milldam full of water possesses energy which, when the 
sluice is opened, drives the mill wheel. Energy in this quiescent 
condition is called "potential" energy. When we actually 
lift the weight or pull the oar, when the watch spring uncoils 
or the water flows out of the milldam, the potential energy 
becomes active or " kinetic," from the Greek word meaning 
" motion," as in the word " kinematograph " or moving picture. 
Heat being a mode of motion is, therefore, kinetic energy. 
Coal contains a store of potential energy, because the carbon 
which is its principal constituent unites under certain conditions 
with the oxygen of the air yielding kinetic energy, as heat 


and light. This heat can transform water into steam, which 
drives the piston up and down the cylinder of an engine. This 
mechanical kinetic energy may in turn be employed to drive 
the electrical machine called a dynamo and so produce elec- 
trical energy, which again causes an electric lamp to glow, 
giving rise to the form of radiant energy we call light. 

Every time one form of energy is turned into another it is 
done at a loss. Thus the whole potential energy of coal is not 
available for transforming water into steam, nor is the whole 
expansive power of steam spent in moving the piston. All the 
mechanical power of the piston does not go to the turning of 
the dynamo, and all the electrical power of the dynamo is not 
converted into light. The potential energy of our arm is not 
entirely expended in pulling the oar, for we ourselves get hot 
with the exertion, and such heat is certainly not helping to 
drive the boat through the water. There is, in short, a loss of 
energy at every step, and the problem for the engineer is to 
devise a machine that may reduce these successive losses to 
a minimum i.e., to invent the most economical engine possible. 
Every form of energy is, in the long run, reducible to heat; this 
heat is eventually given off into space, and we cannot catch it 
again and make use of it. It is not actually lost or destroyed, 
but simply becomes unavailable by dispersion. 

Thermo-dynamics. Obviously it is important for us to 
know how to measure energy in order to be able to draw up a 
debit and credit account, so to speak, to see how much energy 
we may use and how much we are losing. It would also be 
useful to know how much there is of it and what becomes of it. 
This whole subject, a vitally important one to science, is called 
thermo-dynamics, and with it we link the names of Carnot, 
Joule, Helmholtz and Thomson (afterwards Lord Kelvin), four 
of the great physicists of the nineteenth century. 

The Ideal Engine CARNOT. Sadi Carnot was the son of the 
noted Revolutionist, Nicholas Carnot, who took so great a part in 
the organisation of the Republican armies of France during the 
political disturbance that preceded the rise of Napoleon. Sadi 
was born in 1796, but died of cholera at the early age of thirty- 
six. Short as his life was he found time to produce a work on 


the " Motive Power of Heat " that is always regarded as the 
foundation stone of thermo-dynamics. 

Carnot set out to find how much mechanical work it was 
possible to get out of an ideal steam engine i.e., one in which 
there was no loss of energy at all. Such an engine does not 
exist, for were we to supply it with a certain amount of energy, 
which we shall call x, and get out of it a certain amount of 
mechanical work, jy, and a certain amount of heat, z t and if 
we could collect z once more and resupply it to the engine to be 
turned again into mechanical energy, we should come nearer 
to solving the problem of perpetual motion than did any of 
the cranks who puzzled their brains over the subject in the 
sixteenth and seventeenth centuries. In an ordinary railway 
locomotive by far the greater part of the energy of the fuel ist 
lost, and only a relatively small balance is left to pull the train; 
The bigger that balance is the more power we get out of the 
fuel, the lower the cost of running the train, and, incidentally, 
the cheaper our railway tickets ! Perhaps our far-o descend- 
ants may devise some method of utilising the wonderful sub- 
stance, radium, which modern physicists tell us is one of the 
unstable elements that constantly evolves energy and yet lasts 
thousands of years. 

The Mechanical Equivalent of Heat JOULE. Carnot's 
work was entirely theoretical and mathematical, and so we need 
not discuss it, but we must notice that he claimed that it was 
possible not only to change mechanical energy into heat but 
also heat energy into mechanical. What the value of the one 
was in terms of the other he did not try to estimate; that was 
accomplished by Joule. 

James Prescott Joule was a brewer, born in Salford in 1818. 
In his early years he was more or less of an invalid, and spent 
most of his time at home pondering over scientific problems. 
One of these problems was that left unsolved by Carnot the 
actual value in mechanical work of a unit of heat. This he 
solved in the following way (Fig. 61). The figure is simplified 
as much as possible so as to show the general principle only. 

A is a cylinder filled with water, in which revolves a series 
of paddles, B, attached to the axle, D. The cylinder also con- 


tains a thermometer, C. The top of the axle passes through a 
roller, F, which may be fixed to or released from the axle by 
the pin, E, Round the roller is wound a cord which passes 
over the drum, H, and on the same axis is another smaller drum, 
I, also having a cord wound round it, ending in a i-pound 

weight, J. By winding up the 
drum, I, the cord is unwound from 
the drum, H, and thus the weight, 
J, is lifted to the top of the scale, 
K. When J is released, by its 
fall, in obedience to gravity, it 
causes the roller, F, and conse- 
quently the paddles inside the 
cylinder, to revolve. When the 
weight, J, has reached the ground, 
the pin, E, is released and the roller 
is rewound without disturbing the 
water or the paddles. Owing to 
the beating of the paddles on the 
water the temperature of the water 
is raised and the rise is recorded 
by the thermometer, C, while the 
scale, K, indicates the number of 
feet the weight has fallen in a 
given time. It will be seen, there- 
FiG.6i.-~ JOULE'S APPARATUS FOR fore, that the amount of actual 

DETERMINING THE MECHANICAL wor k done by the falling Weight can 

EQIHVALENT OF HEAT. ^ meaSTjred in terms o f heat- 

After making allowances for friction, cooling of the cylinder, 
and so on, Joule was able to say how much mechanical energy 
had to be expended in order to raise the temperature of I pound 
of water i F. He found that to do this the weight must fall 
722 feet. (The number is now determined as 778 feet.) Con- 
versely, the energy sufficient to raise I pound of water i F. 
would lift a i-pound weight 778 feet. The unit, viz., the work 
done in lifting i pound a foot, is called a " foot-pound/' 

The First Law of Thermo-Dynamics. Of course, at every 
step there is a loss of energy in the form of heat, but these losses 


Joule, as we have said, allowed for in making his calculations, 
given in his paper published in 1843. When stated in general 
terms we get what is known as the first law of thermo-dynamics 
viz., when, during any transformation of one form of energy 
into another, heat is produced, the amount is always the same 
for the same amount of energy, whatever its form, and if heat 
disappears, exactly the same amount of some other kind of 
energy will appear in its place. 

All this work of Joule's and much more that we must pass 
over led to the discovery of two great generalisations the laws 
of conservation and of dissipation of energy. The law of 
conservation of energy means that- the sum total of the energy 
of the universe is a constant quantity. We cannot destroy 
energy and we cannot create it, we can only turn it from one 
form into another. It may be potential or it may be kinetic, 
but all the potential energy plus all the kinetic energy is a 
constant quantity. This fundamental law was expounded at 
great length and very brilliantly, in 1847, b Y the German 
physicist, Helmholtz. "We cannot create mechanical force/' 
he says, " but we may help ourselves from the storehouses of 
Nature. The brook, the wind which drive our mills, the 
forest, the coal beds which supply our steam engines and warm 
our rooms, are the bearers to us of the small portion of the great 
natural supply which we draw upon for our purposes/' 

The Second Law of Thermo-dynamics. But there is 
a second law of thermo-dynamics which was hinted at by 
Carnot and expressed by Clausius, professor of physics at 
Bonn, and who was a contemporary of Joule. This law states 
that the work-efficiency of a reversible engine depends on the 
temperatures at which it takes in and gives out heat ; from which 
it follows that heat cannot be conveyed from a colder to a 
warmer body. We can raise the temperature of water in a 
kettle to 100 C., but we cannot transfer any of that heat to 
the fire. " Of course, everyone knows that/' it may be said, 
but, a century ago, everyone did not know it, nor did they see 
where belief in such a law must lead them. When heat is 
supplied to an engine it is impossible to use it all in the per- 
formance of mechanical work, as Watt realised. The heat 


that is not used in moving the piston to and fro escapes and 
warms all the articles round about, which pass on the heat to 
the air and the walls of the engine-room, and so to the exterior. 
The heat is thus diffused or dissipated; we can make no use 
Df it, and are quite unable to collect it again in a condensed 
form for work. 

The law of dissipation of energy was explained for the first 
time by one of our most brilliant physicists, Sir William Thomson 
(afterwards Lord Kelvin), in 1852. If we regard our earth as a 
gigantic clock, kept going by its stores of energy, and if these 
stores are constantly being degraded into the form of generally 
diffused heat, then, in time, the clock must run down. It 
certainly is running down, but, even if it received no new 
supplies, the internal heat will last many millions of years yet. 
On the other hand, its surface is constantly receiving new 
supplies from the sun, and these supplies, as we shall find by 
and by, are, in part, being stored up by plants in the form of 
wood and other organic materials, to be used as fuel or to main- 
tain life. Rivers flow down to a uniform level the ocean 
and thus are losing all the potential energy they temporarily 
acquire when they gather into lakes, and all the kinetic energy 
they release when they are actually in motion. But at the same 
time the sun is constantly causing evaporation of the surface 
waters everywhere, and the vapour so produced condenses into 
clouds which, when they precipitate rain or snow over the land, 
keep the rivers flowing and the lakes at a fairly uniform level, 
and so maintain the supply of energy. 

It would therefore appear that we are, in the long run, 
directly or indirectly, entirely dependent for all our supplies 
of energy on the sun. The sun is a great fiery ball of enormous 
size, about a million times the volume of our earth, with a 
surface temperature of about 6,000 C. and an internal tempera- 
ture of something like 40,000,000 C. But Sol himself is losing 
energy at a stupendous rate, as we shall see when we study the 
marvellous advances in astrophysics during the present century. 
We are thus forced to the conclusion that, unless there be some- 
thing to compensate for all this loss, the whole universe is drifting 
slowly, though very slowly, into a cold state. But now we are 


getting out of our depth, and had better return to the natural 
phenomena taking place around us and the laws we can deduce 
from their observation, and leave these vaster problems to philo- 
sophers who devote themselves to what the Greeks called meta 
ta phusica, the things that come after physics, or metaphysics 

(see pp. 454, 455). 


We must now turn to the subject of electricity, in which such 
gigantic advances have been made in our own times. First of 
all, we must see what progress was made after Gilbert showed 
his magnetic machines to Queen Elizabeth, and Guericke spun 
his sulphur ball before the Emperor Ferdinand. 

Conductors and Non-conductors GRAY. One of the earliest 
of these pioneers, born about a century after Gilbert, was 
Stephen Gray, a master at Charterhouse School. This inquisi- 
tive person's experiments are not only interesting in themselves 
but so suggestive of what was to follow, that some account 
must be given of them. Gray knew all about Gilbert's work 
and proceeded to extend it. He found that if he rubbed a long 
glass tube, it, like amber, attracted pith balls and scraps of 
paper, and that even when he covered the end of it with cork, 
the " vertue " was passed on to the cork. He then used a 
fishing-rod, to the tip of which he attached an ivory ball, and 
found that the ball behaved just like the glass tube. These 
wonders he showed to a friend, who suggested using a wire sup- 
ported at intervals by some silk threads from the roof of a long 
gallery, passing the wire backwards and forwards until it was 
over 300 feet in length. Still the " vertue " passed from the 
rubbed rod, from end to end of the line. In one of these experi- 
ments the silk threads broke under the weight, so they substi- 
tuted metal wires as suspenders, but, no matter how much they 
rubbed the glass at one end, the " vertue " never reached the 
ball at the other. What was the matter ? Why did not the 
" vertue " travel as before ? Suddenly it flashed on them 
that perhaps the reason was that they had used silk and not 
wire in their previous experiments. So thicker and stronger 
silk supports were employed, and the length of their line was 
increased to 650 feet. Then the " vertue " passed from end 



to end of the line quite easily. Here, though Gray did not 
know it, was the germ of the electric telegraph. Gray was 
greatly excited over this discovery, and leaving, as he says, 
his friend to electrify " a hot poker, a live chicken, a large map, 
and an umbrella/' he rushed back to Charterhouse and electri- 
fied his boys in more senses than one by suspending them 
by silk cords, touching their feet with an electrified tube, and 
making bits of paper fly against their faces ! 

A Frenchman, called Charles Dufay, learned of Gray's 
experiments and repeated them, but in doing so he discovered 
certain new facts. He found that metals could be electrified 
also, provided they were held in glass supports, insulated as 
we would say, so that the " vertue " did not pass away to the 
ground from them through the body of the holder; and thus he 
was led to distinguish conductors from non-conductors. But 
he went further; he showed that while a rubbed glass rod 
repelled a film of gold leaf, a rubbed piece of amber attracted 
it, and so he announced that there were two kinds of electricity, 
one of which he called " vitreous " and the other " resinous/' 
and that two bodies, each charged with either vitreous or 
resinous electricity, repelled each other, while if one was vitreous 
and the other resinous they attracted each other. Dufay's two 
kinds of electricity are what we know as positive and negative. 
The Leyden Jar. About the same time two Dutchmen, 
Cunasus and Peter Musschenbrock of Leyden, made a discovery 

of considerable importance which 
resulted in a piece of apparatus 
known as a Leyden jar. In its 
modern form it consists of an open 
glass cylinder, A, lined inside and 
outside with tinfoil, B, C (Fig. 62). 
Within the jar is a rod touching 
the inside lining and ending in the 
knob, D. The inside foil is insu- 
lated from the outside foil by the 
glass A, but the outside is in con- 


tact with a table on which the apparatus stands, which acts 
as conductor to the earth. If D be put in contact with a 


generator of electricity, the tinfoil C becomes "charged," 
and if we take a metal fork with two prongs, F and G, united 
to a glass handle, E, and, while touching the outer tinfoil with 
G, bring the other prong, F, close to D, we shall get a spark 
between them, when F and D are sufficiently close together. 
If the original charge supplied to C is positive, B becomes 
charged with negative electricity by what is called induction. 
When a number of these jars are united in series and properly 
insulated on glass, a well-marked electric* shock can be ob- 
tained, which, when passed through the human body, used to 
create great wonderment, not to say fear, among the country 
yokels at fairs where the travelling conjurer gave his per- 

Lightning Conductors. The well-known American states- 
man and scientist, Benjamin Franklin, noticed the spark 
between the knobs F and D in the Leyden jar. and, in 1749, 
concluded that a flash of lightning was simply a spark, though 
on a very much larger scale, between two oppositely charged 
clouds, and, by means of a kite carrying a wire, he attempted to 
draw off what he called the " electric fluid " from the cloud. 
This dangerous experiment was a complete success, and now 
copper lightning conductors are fixed to every lofty building to 
guide the destructive charge by the easiest pathway to the earth, 
where it is lost and becomes harmless. 

While Franklin was thus risking his life in drawing lightning 
from the sky, and in doing so conferring a boon on mankind, 
two Italians were beginning a new chapter in the history of 
electricity. These were Luigi Galvani, professor of anatomy 
in the University of Bologna, and Alessandro Volta, professor 
of physics in the University of Pavia. Galvani was a man of 
some note in the science of biology, for he added to our know- 
ledge of the structure of the ear and of the anatomy of birds, 
but it is to his discoveries in electricity that he really owes his 
fame. We still speak of a " galvanic battery/' just as we com- 
memorate Volta's name in the " voltaic pile " and in the term 
" volt/' meaning a unit of electric pressure or potential, 

Animal Electricity GALVANI. As is frequently the case in 
relation to great men of past generations, stories get attached to 


their names, like barnacles to an old hulk, but in very many 
instances these turn out to be mere fables. The story of how 
Galvani came to make his discovery of " animal electricity " 
has apparently a germ of truth in it. Signora Galvani, it is said, 
was preparing a soup from frog's legs, while one of her husband's 
pupils was turning an electric frictional machine near by. The 
lad accidentally touched one of the legs with a knife he had in his 
hand and was startled to see the leg move. The lady informed 
the professor of this apparent resurrection, and he at once 
began to investigate the curious phenomenon. He prepared 
a number of frog's legs to experiment upon, and hung them 
by copper hooks to an iron balcony. To his surprise, whenever 
a leg was blown against the balcony, it gave a convulsion, and, 
since there was no electric machine anywhere near, he came to 
the conclusion that there was an " electric fluid " in the leg 
that made itself evident whenever the copper hook was brought 
in contact with the iron through the frog's leg. The leg was 
said to be " galvanised into action," a phrase we still use when 
we speak of spurring on some slow or sleepy worker. 

Voltaic Battery VOLTA. Volta heard of Galvani's discovery 
of " animal electricity," as it was called, but, on repeating the 
experiment, decided that the frog's leg had nothing to do with 
the electric current, and that the same result could be obtained 

by using the metals alone with- 
out the intervention of the 
animal. So a controversy arose 
between the two professors, but 
before it ended Galvani died, 
leaving Volta to work out the 
problem by himself. Long after- 
wards, with the aid of a delicate 
instrument, called a " galvano- 
meter " (note Galvani's name in the title), the existence of these 
electric currents was proved to exist not only in the frog but in 
all living organisms. 

Volta, being convinced that the electric current was pro- 
duced by the contact of two metals, proceeded to make what 
we call a " voltaic battery " by placing strips of copper and 



zinc in glass jars containing brine, arranged in series (Fig. 63). 
When he united the copper strip, C, with the zinc strip, Z, and 
so through the series, he obtained an electric current whenever 
he joined the last copper strip with the first zinc one. 

Thus by the end of the eighteenth century quite a number 
of facts about magnetism and electricity had been accumulating, 
and what now began to exercise people's minds was whether 
there was any connection between them. 

Again and again in the history of science a great discovery 
is made not by planning out beforehand the end to be gained 
and then labouring to reach that end, but by mere accident a 
lucky shot. In the case of a possible connection between 
magnetism and electricity, it was pure chance that gave the 
key and led to the belief that the two sets of phenomena were 
intimately related to each other. 

Electro -magnetic Induction OERSTED. In the year 1819 
Hans Christian Oersted was professor of physics in the University 
of Copenhagen, and on one occasion he was experimenting with 
a galvanic battery in his laboratory. On the table there was 
also a compass which was pushed by accident close to a bar 
of copper through which it was intended to send an electric 
current. It chanced that this bar lay parallel with the compass 
needle, but naturally Oersted paid no special attention to this 
detail. When he sent the current through the bar, to his 
amazement the needle swung round and placed itself at right 
angles to the direction in which the current was flowing. 

Some months afterwards a French physicist, named Arago, 
gave an account of Oersted's discovery to the Academy of 
Sciences, and it chanced that among the audience was a man 
called Ampere, who was born at Lyons in 1775, and who, after 
a somewhat disturbed youth for these were the stormy days 
of the French Revolution, during which his father had been 
guillotined became professor in the Polytechnique at Paris. 
Amp&re at once saw the possibilities of Oersted's discovery, 
and, after a few days' work, was able not only to confirm it 
but to extend it greatly. He found that when the current 
in the conductor was flowing from south to north, and the 
needle of the compass lay below the conductor, the north pole 



of the needle swung to the west, but when it lay above the con- 
ductor it swung to the east (Fig. 64), 

If we place the north pole of one magnetic needle near the 
same pole of another needle they will repel each other, and the 
south poles are equally unfriendly, but the north and south 

poles are attracted to each other. 
Ampere's experiment showed that 
the electric current induced mag- 
netic action round it, and from that 
he argued that two parallel electric 
wires would act magnetically on 
each other also. So he placed two 
rods side by side, both of them free 
to move, and sent a current through 
them in both cases in the same 
direction. "The wires at once came 
together; but when he sent the cur- 
rent in one direction through one 
wire, and in the opposite direction 
through the other, they repelled 
each other. Ampere also measured 
A, needle below; B, needle the stre n g th of the currents, and 

above the conductor. 

deduced the amount of attraction 

or repulsion between them and expressed it in a formula. Thus 
if two wires are at a certain distance apart, d t and if the 
strength of one current be c, and if the other c 1 , then the force 
of attraction or repulsion, /, can be measured by multiplying 
c by c 1 and dividing the product by the distance squared 

. - cc* 

The Galvanometer. It was hinted above (p. 136) that 
Galvani's " animal electricity " would be demonstrated later 
when the galvanometer had come into use, and here we have 
the germ of the invention. The instruments now employed 
are constructed on various patterns, but all we need know is 
the principle on which they work. If a wire be bent into a loop 
(Fig. 65), and a magnetic needle be placed within the loop, the 
needle will be under the influence of any current passing through 

A B 


S-N, direction of the current; 


the wire, and will thus be actuated by two forces, first, its own 
tendency to point north and south, and, second, the current 
enticing it to point east and west. The stronger the current the 
more the needle will vary, and 
the angle between the needle 
at rest and after it has been 
acted upon by the current gives 
us a figure by which to calcu- 
late the strength of the current, 
and a standard unit of fixed 
quantity is called an " amp&re." 
But the idea of surrounding 

themagnetwithacoilofwireled FlG . 65 ._ GALVANOMETER . 
Ampere to something else. He 

wound a long wire round a bar of steel and sent a current through 
the wire; the bar retained the magnetic charge and became a 
magnet, although when he replaced the steel bar with one of 
soft iron, the latter lost its magnetism the moment the current 

was cut off. The steel bars were 
termed " electro-magnets/' see- 
ing that they had been made by 

^ ( the use of an electric current. 

FIG. 66. ELECTRO-MAGNET. In 1836 Ampere, who had 

long suffered from tuberculosis, 

started on a cruise in the Mediterranean, but before he reached 
Marseilles he succumbed to his malady, leaving behind him a 
record of domestic troubles bravely borne, and of scientific 
achievements which placed him in the front rank of the pioneers 
of electrical science. 

SIR HUMPHRY DAVY. It has already been noted how men 
renowned in one branch of science very frequently made 
their mark in related sciences as well. Black, for instance, 
though a professor of chemistry, yet established a great physical 
law that of latent heat. Sir Humphry Davy, a chemist 
of no mean order, was also a physicist of first-class rank. He 
was born in Penzance in 1778, and educated there and after- 
wards at Truro. Even in his early boyhood he showed signs 
of genius though of a peculiar sort, not in the least like what 


one would expect to find in a future leader in science. Before 
he was nine years old he recited stories and poems to his 
schoolmates, entertaining them vastly, no doubt, but hardly 
helping them in their ordinary tasks. 

Davy's father died when he was sixteen, and Mrs. Davy 
was left with five children, of whom Humphry was the eldest. 
The widow was at first rather poorly off, and for some years 
had to help in running a millinery shop, but things improved 
after she fell heiress to a small estate. Meanwhile Humphry 
became an apprentice to a Mr. Borlace, a surgeon-apothecary 
in Penzance, and while studying his profession in the country 
pharmacy, he spent all his leisure in reading philosophy, history 
and poetry, writing essays on all sorts of subjects, and even 
composing verses, which showed considerable merit. 

Before he was twenty he began to devote himself to the 
study of chemistry, and fortunately used as his textbook the 
famous work that had just come from the pen of the great 
French chemist Lavoisier. Davy, with Lavoisier's book as 
his guide, started experimenting on his own account, using 
the materials and rather crude apparatus in Mr. Borlace's 
surgery. Some of these " researches/' as he rather grandilo- 
quently called them, fell into the hands of a Dr. Beddoes of 
Bristol, who thought so much of them that he invited Davy 
to be superintendent of an institute he had founded in that 
city, the object of which was to study the effects of different 
gases on the human body. Davy accepted the offer, and 
entered heartily into the various chemical problems that arose 
from time to time. 

In 1799 Count Rumford had founded the Royal Institu- 
tion in London. He had just brought out his theory that 
heat was not a substance but a " mode of motion/' and Davy 
s had supported that view by his experiment of melting two 
blocks of ice by simply rubbing them together. Rumford 
was naturally impressed, not to say pleased, with this fresh 
proof of the truth of his thesis, and having heard glowing 
accounts of Davy's promise from Dr. Beddoes at the Bristol 
Institute, he invited Davy to come to London and become a 
member of the staff of the new college. This offer came to him 


in 1801, and, with his appointment, began the long and brilliant 
career that marked him out as the foremost experimenter 
and teacher of his age. 

Guericke's air-pump (p. 47) was by no means a perfect 
instrument, and Boyle's apparatus (p. 49) was not very 
much better, but the work that Davy undertook in chemistry, 
soon after his appointment in the Royal Institution, required 
a far more perfect " vacuum " than could be obtained by either 
of these old appliances, so he started to invent an air-pump 
on a new principle. His idea was that if he replaced" the 
atmospheric air in a bell-jar with another gas, the last traces 
of which could be removed by chemical means, he might obtain 
what he desired a perfect vacuum. He knew that carbon 
dioxide was greedily absorbed by caustic potash, so he filled 
the jar with that gas, leaving in it a vessel containing potash. 
He then pumped out as much of the gas as he could in the 
ordinary way; what was left over he considered the potash 
would account for. By this means he obtained a far closer 
approach to a vacuum than did any of his predecessors. 

Count Rumford had seen to it that his new institute was 
not merely a building with lecture-rooms and laboratories, but 
that it was also well supplied with appliances wherewith to 
demonstrate the subjects taught, so that when Davy began 
his work he found, among a variety of other apparatus, a 
battery far more powerful than existed anywhere else. 

The Electric Arc. When we were considering the structure 
of a Leyden jar (Fig. 62) we noted that when the ends of one 
of the prongs of the fork 
were brought near the 
knob of the central rod 

in contact with the FlQ 67> __ ARC BETWEEN CARBON TERMINALS. 

inner tinfoil, a spark ap- 
peared between the knob and the prong. Davy studied the sparks 
given out by the two terminals of the great battery in the Royal 
Institution, and found that as he drew the ends of tie wires apart, 
not one but many sparks seemed to jump from one terminal to the 
other, and that both terminals became very hot. After trying 
many substances, he found that when the terminals were made 


of carbon he obtained a brilliant white light, and that the 
positive pole was worn away by the passage of carbon particles 
across the gap to the negative pole. When the carbon rods 
were placed horizontally the flame resembled a bow which he 
called the "arc," and that the top of the positive pole was 
hollowed out into a cavity, or crater, the walls of which were at 
a white heat. He estimated the temperature of this cavity at 
3,000 C., and found that he could melt in it many substances, 
such as quartz or platinum, that were quite proof against the 
temperature of an ordinary furnace. Thus Davy is responsible 
for the first beginnings of the electric furnace now found in 
every well-equipped chemical laboratory, and for the great 
arc-lamps that illuminate our streets and railway stations. 

Electrolysis. Nearly twenty years previously Cavendish 
had discovered that when hydrogen and oxygen were mixed in 
certain proportions in a vessel, and the mixture ignited, the 
result was the disappearance of the gases and the formation 
of a dew on the vessel's wall which he found to be water. It so 
happened that two other physicists, Nicholson and Carlisle, 
while experimenting with a voltaic battery, found that, when 
the ends of the two wires were placed in ordinary water, 
bubbles of gas appeared round each of them, which were 
hydrogen and oxygen respectively. They also noticed that the 
water at the positive terminal became acid and at the negative 
terminal alkaline. These peculiar results they were, however, 
unable to explain. Davy was very much struck by these facts, 
and employed his genius to solve the riddle. At first he fancied 
that the acid and the alkali might have come from the vessel 
in which the experiment was carried out, so he used, first, agate 
and then gold cups to hold the water and the terminals, but 
the acid and alkali still made their appearance. Suspecting 
that the water he had distilled in the ordinary way might have 
carried some salts with it, he next used water that had been 
obtained by natural distillation or evaporation, and found that 
although the alkali was as prominent as ever, the acid, though 
still present, was much weaker. Had the air anything to do 
with it ? To answer this question he used his new method 
if producing a vacuum, and carried out the operation under 


the bell-jar of the air-pump. He then found that almost pure 
oxygen came off at one terminal and almost pure hydrogen at 
the other, and further that as the gases increased in volume the 
water in the cup decreased, thus completely confirming 
Cavendish's discovery of the composition of water. 

But this was by no means all. If water could be decom- 
posed by an electric current, he asked himself, why not other 
substances ? The first materials he experimented with were 
potash and soda, which at that time were considered to be ele- 
ments. Davy melted some caustic potash in a platinum spoon 
and connected the spoon with the terminals of his great battery, 
when, to his amazement, the liquid potash began to bubble, 
and bright droplets, like globules of mercury, appeared on the 
surface. He obtained a similar result when he treated caustic 
soda in the same way. He had obtained from these so-called 
" elements " two new metals, so that potash and soda must be 
compounds. The shining globules he christened respectively 
potassium and sodium. His brother, who was present when 
this discovery was made, says that Davy was so excited that 
" he actually bounded about the room in ecstatic delight/ 1 And 
well he might, for he had not only added two new elements 
to the ever-growing list but he had also discovered a new 
method of analysis, which was to prove not only of immense 
value in chemistry but also was destined to be the basis of 
great industrial undertakings, such as electroplating and 
electrotyping. The method is now known as "electro- 
lysis/' or " releasing by electricity/' 

Here we may pause for a moment in the story of Davy's 
discoveries and consider what electrolysis has meant to the 
silversmith. The handles of forks and spoons and other silver- 
like articles have certain marks stamped upon them, and one 
of these is " E.P.," which stands for " electroplate/' meaning 
that the spoon is not solid silver but a far less costly metal 
pewter, nickel or some alloy coated with silver. How is this 
accomplished ? By a method based on Davy's discovery. 

Fig. 68 shows how a pewter teaspoon, made from an alloy 
of tin and lead, may be made to look like silver. A is a bath con- 
taining a weak solution of a compound of silver. Supported on 


the edges of the bath are two rods, B and C, C being connected with 
the positive pole of the battery and B with the negative pole. 
D and E are crossbars, D resting on the positive rods and E on 
the negative ones. From E depends the spoon to be electro- 
plated, H, dipping into the solution, G, and from D hangs a 

plate of pure silver, F. The 

D current enters at C, passes 

along D, and reaches the sil- 
ver plate, F, and the solution 
of the silver salt, G. From 
thence it passes through the 
spoon, H, and then through 
the crossbar E to B. So 
FIG. 68. ELECTROPLATING. much for the apparatus ; next 

a word as to the method. 

The spoon is first of all boiled in soda to remove any grease ; 
it is then thoroughly washed and dipped in a solution of nitric 
acid. Just before placing it in this bath, it is plunged for a 
moment in a solution of a salt of mercury, which aids the 
subsequent silvering process. Technically this is called 
"quickening." The spoon is then suspended by wires from 
the crossbar E, and the current is turned on. The silver in the 
solution G in the bath separates out from the salt and is 
deposited on the spoon, and an equal amount of silver takes 
its place, dissolving from the plate F, so long as the current 
is maintained. The time taken in the process depends on the 
thickness of the silver coating required and the nature of the 
article to be coated, but usually several hours are needed to lay 
down a layer of silver as thick as the page of this book. When 
the coating is completed, the spoon is washed and polished 
with wire brushes, again washed in boiling water, and dried in 
hot sawdust. 

Electrotyping is another important application of Davy's 
discovery, without which this book could not have been printed 
and illustrated, but to describe that process also would be 
to take up too much of our space. Let us rather return to 
the Royal Institution laboratory and see what other important 
work in physics was carried out by its distinguished professor. 


Davy's Safety Lamp. In the early years of last century 
explosions in coal mines were of very frequent occurrence, and 
often attended by great loss of life. Naturally this was a 
matter of the gravest concern to the public, for it made coal- 
mining a very dangerous occupation, and discouraged men from 
risking their lives in underground workings in order to supply 
the fuel needed not only for household fires but also for the 
furnaces of the great iron and steel trades, and the various other 
factories then springing up all over the kingdom, wherever coal 
was to be had most cheaply. To lessen the risk to the miner 
was also to make him contented with a lower rate of wages, to 
cheapen coal, and so to benefit both the general public and 
the manufacturer. 

These disastrous explosions that made the miners life a 
constant terror are chiefly due to one or other of two causes: 
the one is what the miner calls " fire damp/' and the other is 
coal dust. " Fire damp " is a gas, often called " marsh gas/' 
because it may be seen bubbling up when a stagnant pool is 
disturbed. It is a compound, of carbon and hydrogen, also 
known as methane, and is highly combustible; when it explodes 
with sufficient air it produces carbon dioxide and water. 
Carbon monoxide, however, a deadly poisonous gas, frequently 
appears, as well as carbon dioxide in mine explosions, giving 
rise to what the miners call " choke damp " or " after damp " 
viz., a mixture of carbon monoxide and carbon dioxide. In- 
cidentally it may be noted that the word "damp" is Old 
German for an exhalation or vapour, not necessarily aqueous. 
Clearly both these gases deprive the air of the gas, oxygen, 
which is essential to respiration, while one of them adds a 
deadly poison to the atmosphere. At the same time the actual 
explosion may cause great damage to the walls and roofs of the 
underground passages, so blocking the pathways of escape. 

Fine coal dust is also very explosive, as, indeed, are all com- 
bustible bodies when in the form of a very fine powder. 
Obviously miners required lamps to enable them to carry on 
their work, and yet a bare flame might produce an explosion 
from either cause, with consequent disaster. 

The year 1815 was a particularly bad one for explosions 


in the coalfields, and the harassed mine-owners wisely appealed 
to Davy for help. Davy at once responded, and in a couple 
of weeks produced his famous " safety lamp/' the use of which 
has saved thousands of lives. The principle on which it is con- 
structed is remarkably simple. Obtain a small piece of fine 
copper wire gauze and hold it over a gas-jet; turn on the 
gas and light the jet below the gauze. The flame does not 
pass through the gauze, but rises and falls as the gauze is raised 
or lowered. Arrange the gauze as before and turn on the gas, 
but this time apply the match above the gauze; now the flame 
will appear above it, but not below it. The gauze may be 
raised an inch or two, but no flame " strikes back " to the 
burner below. The explanation is that the gauze is a good 
conductor of heat, and the large surface of wire exposed allows 
the heat to be well distributed, and thus the temperature is 
brought down below that at which the gas takes fire. The 
gas, of course, passes through the gauze readily enough, and 
so it is possible to light it above the wire gauze. 

Davy recommended that all oil lamps used in mines should 
be encased in fine wire gauze (Fig. 69, GA), for although " fire 
damp " might enter along with air and might catch 
fire inside the lamp, the flame would not strike back 
and ignite the " fire damp " outside. Besides, the 
ignition of the " fire damp " inside the lamp would 
GA act as a warning that this colourless and odourless 
explosive gas was present in quantity in the work- 
ings. Where electric light is not used in the mines 
nowadays, the safety lamps are so constructed 
G that the miners cannot open them; even if they 
try to do so, the lamps at once go out. The base 
of the lamp has usually a protected glass window 
(1 ^ (Fig. 69, G) to provide adequate light. 

FIG. 69. Davy's safety lamps were at once brought into 
SAFETY LAMP. ; , i , ox 

use, and proved a complete success. Save tor a 
service of gold plate with which the grateful mine-owners pre- 
sented him, he made no money out of his invention, although 
had he patented it he might have made thousands of pounds a 
year from royalties. A friend took him to task for this over- 


generous behaviour, but Davy replied: "No, my friend, I never 
thought of such a thing. My sole object was to serve the cause 
of humanity, and if I have succeeded, I am amply rewarded in 
the gratifying reflection of having done so/' 

All these labours in physics and chemistry undermined 
Davy's health very considerably, and at one time it looked as 
if his illness might prove fatal. But he recovered and resumed 
his labours, though in a less strenuous fashion. In 1812 he was 
knighted, and the day after receiving the accolade he was 
married. The lady of his choice was a wealthy Scottish widow, 
who seems to have been the chief agent in making him forsake 
his beloved laboratory for a long continental tour, during which 
he met many of his famous fellow-workers in science in France, 
Germany, and Italy, For his invention of the safety lamp he 
was made a baronet, and, finally, President of the Royal 
Society, the highest scientific honour he could receive. 

He had scarcely returned to work when he had an attack 
of apoplexy, and had once more to spend the winter abroad. 
He never returned, but died in Geneva in 1829, at the com- 
paratively early age of fifty-one. On his tomb is engraved 
" Summus arcanorum naturae indagator " (the greatest ex- 
plorer of the secrets of nature), and he well merits praise 
so high. 

Looking back over the life-stories of the great men of the 
past, it is surprising to note how few of them had parents who 
could afford to start their sons in life with a good education 
or to provide them with the wherewithal to gain it for them- 
selves. Kepler was a potboy in a country inn; Galileo was an 
apprentice to a draper; even Newton was only the son of a 
small farmer; Herschel was a bandsman in a German regiment; 
Desmarest could hardly read when he was fifteen, and was fed 
and taught by the charity of the monks of Troyes; Watt was 
the son of a bankrupt shopkeeper; and even Davy, who died a 
baronet, loaded with all the honours his grateful countrymen 
could pour on him, began life as a bottle-washer in a country 
apothecary's shop. It is not necessary, therefore, to be born 
with a silver spoon in one's mouth a wooden ladle may do as 
well or to have been trained in the best school and university 


in order to become a brilliant inventor or a distinguished man 
of science. One of our great writers, Anthony Trolope, 
makes a character in one of his novels pronounce the secret of 
success, "It's dogged as does it; it ain't thinkin' about it." 
Each of the great men we have mentioned, poor and, in most 
cases, ill-educated as they were, had a fixed idea in his mind 
as to what was to be his life's work, and perseverance and 
hard work did the rest. If this be true of many of the men 
whose acquaintance we have already made, it is doubly so of 
the man whose work we have now to study Michael Faraday. 

MICHAEL FARADAY. Michael was the third son of a black- 
smith who had married a crofter's daughter in Yorkshire, and 
who came up to London in the hope of mending his fortunes. 
But the times were very hard, and food was at a ransom. 
Michael, as a boy of five, had to live on one loaf a week ! His 
father also was in bad health, and the family had at last to ob- 
tain relief from the rates. Michael seems to have mastered the 
" three Rs " at a day school, and spent the rest of his time, 
as he says himself, " at home or in the streets." " Home," 
by the way, was a couple of rooms over a coach-house in a 
mews near Manchester Square. 

In 1804, when he was thirteen years old, he became an 
errand boy to a bookseller, and did his work so well that his 
master made him apprentice to the bookbinding trade without 
asking for a premium. Michael not only bound the books his 
employer gave him to work on, but read many of them as well, 
especially those that dealt with chemistry and physics. By 
the time he was nineteen he had managed to save enough out 
of his wages to pay for attendance at a course of lectures on 
physics, or natural philosophy as it was then called, and wrote 
out his notes of the lectures, illustrated by drawings of the 
apparatus used by the lecturer. 

One of the patrons of the book-shop was a certain Mr. 
Dance, who happened to be a member of the Royal Institution, 
and this gentleman, being struck with Faraday's love of science, 
took the lad with him to hear Sir Humphry Davy lecture. 
Davy was a brilliant lecturer, and young Faraday came away 
enchanted. Bookbinding seemed now a detestable task, and 


Faraday was prepared to do anything that might save him from 
the drudgery of sewing and gumming sheets of paper together 
from morn till eve. But what else could he do ? He had no 
influential friends, not even Mr. Dance, for, having finished his 
apprenticeship, he had left the employ of the bookseller where 
he had first met him. But Faraday was not to be beaten. 
He took upon himself to write direct to Sir Humphry Davy, 
enclosing the notes he had taken of his lectures by way of 
showing what he could do. Imagine his delight when, a few 
days later, a carriage drove up to his door and a footman 
handed in a letter from the great man himself asking him to 
call at the Royal Institution. After the interview he was 
offered the post of laboratory attendant at a weekly wage of 
255., together with a bedroom in the Institution. Now his 
foot was on the first rung of the ladder, a ladder he never 
ceased ascending, until there were no more steps left for him 
to climb. He soon proved to Davy that in him the latter had 
found not only a keen and even enthusiastic worker but a genius 
of the highest order, and not many weeks passed before Davy 
made Faraday his assistant and private secretary. Some time 
afterwards, when Davy was asked what he considered to be his 
greatest discovery, he promptly replied, " Michael Faraday. 11 

After his marriage in 1812, Davy set out on his continental 
tour, and he invited Faraday to accompany him, and thus the 
young scientist was given the opportunity of seeing something 
of the world and, better still, of meeting some of the great men 
of science in France and Italy. In spite of the advantages he 
thus gained, Faraday seems to have had fits of homesickness, 
for he wrote to one of his friends, " I fancy that when I set 
foot in England I shall never take it out again/* and on his 
return journey he wrote to his mother: " At Deal we land on a 
spot of earth which I shall never leave again/' His deter- 
mination to stay at home in future was, however, not main- 
tained, for twice afterwards he spent several months on the 

The teacher in natural philosophy, whose lectures he had 
attended while he was working in the bookseller's shop, had 
meanwhile established a sort of club, called the " City Philo- 


sophical Society/' and of this body Faraday became a member, 
and to it he gave his first public address. Shortly afterwards 
he published, in the Journal of the Royal Institution, a paper 
on the composition of lime, and read another paper to the 
Royal Society on certain new chemical compounds. Faraday 
was now twenty-nine years old, and it was then that he began 
the researches that have made his name famous for all time. 

Oersted, it may be remembered (p. 137), had found that 
when he brought a magnetic needle close to a charged wire, 
the needle moved from its normal north-south position to one 
at right angles to the direction of the current, and Ampere had 
found (p. 137) that whether the north pole pointed west or east 
depended on whether the wire lay below or above the needle. 
Faraday attempted to answer the question why this should 

be so. 

Relation of Magnetism to Electricity. Take an ordinary 
electric battery, and on the course of the wire joining the elec- 
trodes fix a piece of cardboard, on which sprinkle some iron filings 
(Fig. 70). On gently shaking the card the 
iron filings will arrange themselves in con- 
centric circles round the wire, which are 
called "lines of force/ 1 and a magnetic 
needle can revolve round the wire. It oc- 
curred to Faraday that if a magnetic needle 
revolved round a charged wire, why should 
not a charged wire revolve round a magnetic 
needle ? To test this he devised the follow- 
ing experiment (Fig. 71). 

He took two glass cups, A and B, and 
drilled holes through their bases to permit of the passage of the 
wires, J, K, The wire, J, ended abruptly just above the base of the 
cup, A, while K was continued upwards, as E, to the top of the 
cup, B. To the short wire he attached a metal rod, D, so that D 
was free to move in any direction. Above the two cups he fixed a 
rod with two knee-joints, insulated at C from the support, I. One 
leg, F, hung down into the cup, A, while the other had attached 
to it, by a fine wire, a metal rod, G, also free to move in any 
direction. Both cups were filled with mercury, H, The 


current entered by J, passed on to D, through the mercury to 
F, down the hanging rod, G, through the mercury in B and E, 
and escaped by the wire, K. To his intense delight Faraday 
noticed that when the current 
was turned on, the rod, D, re- 
volved round F, while G re- 
volved round E. An onlooker 
tells us that Faraday became 
so excited that he danced round 
the table shouting, " There they 
go ! There they go 1" 

The Induction Coil. In 
order to understand Faraday's 

next discovery one that had I * 


f ar-reaching results it is neces- RoDS> (See Text>) 

sary for us to know the mean- 
ing of certain terms used in electricity. 

Electricity, as we have seen, can flow along a " conductor," 
and some substances, like silver, copper, nickel and graphite, are 
good conductors i.e., allowing the current to flow easily, 
while others will not carry the current at all, such as india- 
rubber, glass, or porcelain. These are used as " insulators/' 
literally " island makers/' forming impassable regions round 
the current. Porcelain cups are used as insulators for telegraph 
and telephone wires, such as may be seen on any telegraph pole. 
If we cover a copper wire with a sheath of rubber, we are really 
forming a tube by which we may transmit a current of 
electricity any distance we please, without any leakage taking 
place on the way. Air is a bad conductor, though not a perfect 
insulator; still it serves the purpose of such in the case of tele- 
graph and telephone wires. 

When a kettle boils, steam is produced, and when the lid is 
lifted off the steam escapes in volumes without appreciably 
pressing on the walls of the kettle. If we replace the lid and 
close the spout, leaving only a small hole in the lid, the steam 
cannot escape as freely as before, and it at once begins to exert 
pressure on the walls of the kettle and on the lid, while some 
of it issues through the hole in the lid with considerable force. 


Thus we may allow steam to escape from the kettle in large 
quantity but at very low pressure, or in smaller quantity at high 
pressure. When an electric current passes along a conductor, 
it may be passing in large quantity at low pressure or in smaller 
quantity at high pressure. This analogy enables us to under- 
stand the difference between " amperage " and "voltage/* 
Amperage corresponds to quantity flow, as when the kettle- 
lid is removed, and voltage to pressure flow or what elec- 
tricians call "tension" (potential) as when steam can escape 
only through the hole in the lid. Hence we may have an elec- 
tric current of high or low amperage and high or low voltage. 
The "ampere'* is a unit of quantity, while the "volt" is a 
unit of tension or electromotive force (potential). 

A current of electricity, however, will travel along a con- 
ductor only from higher to lower potential, forming what is 
called an " electric circuit." This circuit may be broken 
anywhere by introducing into its path a " switch." In 
every house lit by electricity switches are fixed to the walls 
of the rooms, by which it is possible to " turn on " the current 
and so to obtain light, or to " switch it off." If the cap cover- 
ing one of these switches be removed, it will be seen that the 
outside handle operates on a lever whose free end is forked. 
The two prongs of the fork come into contact with two 
" terminals," to each of which is screwed the end of a wire, one 
carrying the current to the switch, the other carrying it away 
from it. Between the two terminals there is a gap too wide 
to allow the current to pass, for air is a bad conductor, until the 
gap is bridged by the forked lever. 

Consider next another analogy. Suppose we attach a 
length of hose-pipe to a water tap in a garden from which we 
may obtain a plentiful supply of water. When the tap is 
turned on, the water flows out freely in a steady stream, and 
if the pressure in the main be sufficient, it may be projected for 
several feet. But suppose we desire to sprinkle the water over 
a flower-bed several yards away without wetting the paths or 
shrubberies lying between the end of the hose and the flower- 
bed, it may happen that the pressure is not sufficiently great 
to force the water so far. To gain our object we attach a nozzle 


with a narrow outlet to the end of the hose, thus reducing the 
quantity of water driven out at a given time, but increasing 
the pressure on the aperture. The force with which the 
water is now squirted out enables it to jump over the intervening 
space. In a similar way, although an electric current carried 
by a wire from some source of supply may be great enough in 
amount i.e., in amperage it may be too low in pressure 
i.e., in voltage and hence it becomes necessary to introduce 
something that will play the part of the nozzle on the hose-pipe, 
to turn amperage into voltage. This apparatus is called an 
" induction coil/' and this was Faraday's next discovery. 

Ampere had found (p. 139) that when he surrounded a bar 
of steel with a coil of wire through which he sent an electric 
current, the bar retained the charge and became a magnet, but 
that if the bar was of soft iron it retained the charge only so 
long as the current was flowing. Electricity could thus be 
turned into magnetism, and Faraday, meditating on this fact, 
asked himself why could not magnetism be turned into elec- 
tricity ? 

In his first experiments he took a ring of soft iron, round 
each half of which he wound wires, X and Y (Fig. 72). One 
end of X was connected with one terminal of the battery, B ; the 
other end, after being coiled 
round one-half of the ring, was 
connected with the other terminal 
of the battery, having on its 
course a switch, S. Similarly the 
other half of the ring had a wire 
wound round it, with a galvano- 
meter, G, on its course. When 

_ .. - , , , FIG. 72. ELECTRO-MAGNETIC 

the switch was closed a current INDUCTION. 

passed through the wire, X, and 

back to the battery, but at the moment of closing the switch 
a current was set up, or "induced," in the wire, Y, and the 
needle of the galvanometer rotated in one direction and 
then came to rest. The same performance on the part of the 
galvanometer needle, though in the reverse direction, took 
place when the switch was opened; the galvanometer " kicked " 


at every " making and breaking of contact/' but showed no 
agitation while the current was actually flowing. Why? 
Faraday was unable to answer that question at the moment, 
but in 1831 he was more successful, and produced an apparatus 
by which an electric current was created by a moving magnet. 
He wound several hundred yards of copper wire round a 
hollow wooden reel (Fig. 73, A), and connected the end of the 

wire with a galvanometer, G. 
He next took a bar magnet, 
B, and inserted it into the 
reel, and at once obtained a 
" kick " in the galvanometer 
needle in one direction, and 
when he withdrew the magnet, 
he obtained a "kick" in the 

FIGT73. ELECTRIC CURRENT INDUCED opposite direction. During the 

BY A MAGNET. ,. ,-, , . -, 

time the magnet was inside 

the reel the needle remained at rest. If, then, a magnetic field 
can be produced, a current will be induced in the conductor 
every time it cuts the lines of force (p. 150) in the field. 

Without going into any detail, the general principle of the 
instrument may be understood from Fig. 74. A is a bar of 
soft iron, or better, a bundle of soft iron 
rods tightly bound together, which, 
when a current is flowing through it, D 
acts as a magnet, and round this bundle 
is wound a long length of thick copper A 
wire, B. (For the sake of clearness the 
wires are very loosely wound, and only 
a few coils are shown.) The end, B, 
is connected with the source of the 
electricity, as is also the other end, C, 
after the wire has formed very many 
coils round the bundle. On the path of 
the incoming current there is a switch, 
S, or something that corresponds to it. 

This is called the " primary coil/' and carries a current of high 
amperage, but of low voltage. It represents the stream of 



water from the garden hose without the nozzle (p. 152). Out- 
side the primary coil, and insulated from it, is wound a much 
greater length of thin copper wire in a far greater number of 
coils, the " secondary coil/' D, E, and its free ends may be 
brought together at F, where there is a gap. 

When the current of high amperage, but low voltage, passes 
through the primary coil, it transforms, as we have seen, the 
bundle of rods into a powerful magnet. The moment the circuit 
is broken at S, the bundle becomes demagnetised, and there is 
immediately induced in the secondary coil a current of low 
amperage but high voltage, sufficiently high to overcome the 
resistance of the air at F, and enable it to jump the gap between 
the two terminals and produce a spark. It represents the nozzle 
fitted to the end of the garden hose that enabled the water to 
be shot out to a considerable distance. If a rapid succession 
of sparks be required, we must " make and break contact " 
at S automatically by machinery. It is by such a piece of 
apparatus that the sparks are produced inside the cylinders of 
a motor-car engine, in order to explode the petrol mixture that 
takes the place of steam, the spark itself being transferred to 
the interior of the cylinder by what is called a " sparking plug/* 
It was thus Faraday's genius that made the motor-car possible. 

The Dynamo. If a magnet be made to revolve inside a coil 
of wire, or, what comes to the same thing, a coil of wire be made 
to revolve between the poles of a stationary magnet, a current 
of electricity is induced in the wire, provided the wire cuts the 
lines of magnetic force at an angle. The current may then be 
collected by brushes, transferred to conductors, and employed 
for all sorts of purposes. This is the principle of the modern 
dynamo, but it is impossible to go into the very complex 
structure of the machine without taking up far more space than 
we can afford. Details must be studied in any one of the 
numerous works on electrical engineering; here we can deal 
only with general principles. 

Measurement of Resistance OHM. In addition to the am- 
pere and the volt there is another electrical term that is constantly 
met with in books on electricity viz., the " ohm/' The word 
is the name of a man, Georg Ohm, who was born at Erlangen, 


in Bavaria, in 1789, the son of a locksmith. His father was a 
poor man we had almost said " as usual " but, fully alive 
to the value of education, he managed to send Georg and his 
brother to the university. After taking his degree, Georg 
became a schoolmaster and finally a lecturer on physics and 
mathematics in the Jesuit High School at Cologne, where he 
laboured for ten years. 

Ohm's first work was to estimate the relative values of 
conductors, and for this purpose he experimented with wire 
made of different materials of the same sectional area, but of 
different lengths. He found that copper was the best conductor, 
and, reckoning its value at 1,000, he placed the other common 
metals in the following order: gold 574, silver 356, zinc 333, 
iron, 175 and lead 97. His figures are not very accurate 
according to our modern estimates, for the relative powers of 
electric conductivity are now given as: silver 1,000, copper 999, 
gold 800, zinc 299, iron 155 and lead 88. His chief error was 
in the case of silver, but the cause of that discrepancy from 
modern estimates was the faulty nature of the wire with which 
he experimented. Silver is the best conductor, but as it beats 
copper only by a mere fraction, and as it is so very much more 
expensive, it is scarcely ever used in electrical machines. 
After much experimental work Ohm was able to announce that 
the conductivity of a wire depended on three things: length, 
sectional area and material. If I represents length, a, sectional 
area, and 5 be a value dependent on the material, then the 
resistance of a wire to the passage of a current, represented by R, 

is expressed by the formula R = -- i.e., the resistance varies 


directly with the length of the wire and the nature of the 
material, and inversely with the sectional area. Again, if C 
be the amount of the current and E be the force that sets up 
the difference in potential that makes the current go (what 
we now call the electromotive force, E.M.F., or E), R be the 
resistance offered by the wire, and r the resistance offered by 

the battery, then C =r - This is known as Ohm's law, and 

a resistance unit is spoken of as an " ohm/" just as a quantity 


unit is an " ampere " and an intensity or potential unit is a 
" volt/ 1 

Electrolysis. It was only natural that Faraday should 
devote some attention to the subject in which his master, Sir 
Humphry Davy, had made his name viz., electrolysis. Davy 
had decomposed potash and soda by means of a voltaic current, 
and obtained the metals potassium and sodium. Faraday 
showed that potash and soda could also be decomposed by 
a current derived from an ordinary friction 
machine. It was in his paper on this subject 
that he introduced the terms now in common 
use. Thus the terminals of the wires from the 
battery are called "electrodes/* the positive 
being the " anode " (Fig. 75, A) or " up end/' 
and the negative the " cathode " or " down 
end/' C, the material split up is the "elec- 
trolyte/' the element passing to the cathode 
the "cation," "ion" meaning rt wanderer/' 
and that passing to the anode the " anion/' 


After many experiments Faraday was able FIG. 75. ELEC- 
to state his law of electrolysis viz., that the 
amount of chemical action taking place in the 
electrolyte depends solely on the amount of the electric current 
passing through it, and, conversely, the amount of decomposition 
of the electrolyte in unit time is a measure of the amount of 
the current. He pointed out that the same amount of current in 
amperes induces different amounts of decomposition in different 
electrolytes, and these amounts he called "electro-chemical 
equivalents/' Thus, if a current of i ampere, acting on a 
compound of silver, deposits 4*025 grams of silver per hour, it 
will deposit ri8i grams of copper from a copper salt in the 
same time, so that 4*025 and ri8i are electro-chemical equiva- 
lents for these metals respectively. 

Action of Magnetism on Polarised Light. We have already 
referred to the curious optical effects discovered by Young and 
Fresnel in relation to crystals of Iceland Spar and tourmalin 
(p. 116), the phenomena of polarised light. Faraday took a deep 
interest in this subject, and endeavoured to find whether 


magnetism had any influence on polarised light, and if so, 

There is one thing that has struck all who have studied 
Faraday's work, and that is his wonderful insight and imagina- 
tion, the almost prophetic power he had by which he was able 
to foresee the outcome of some quite simple experiment he 
happened to be conducting at the moment. When he read his 
paper on " The Magnetisation of Light " at the Royal Society 
he expressed one of these prophecies, and told his audience that 
he believed " that the various forms under which the forces of 
matter are made manifest have one common origin/ 3 and that 
he included light among them. 

The particular experiment of which he then gave an account 
aimed at finding whether an electro-magnet has any influence 
on a beam of light passing near its poles. He used a powerful 
Argand lamp (i.e., one where the wick is in the form of a hollow 
cylinder, so that air is admitted from below, feeding the flame 
both inside and outside), and polarised its light by reflecting it 
from a glass mirror. He then examined it after it had passed 
through the magnetic field. He tried changing the direction 
of the light, and used all sorts of media glass of various kinds, 
Iceland Spar, tourmalin, etc. but every experiment was a 
failure, the magnet seemed to have no effect whatsoever. In 
a final effort he tried a very heavy lead glass, which he had 
used sometime before for another purpose, and, to his delight, 
he got a rotation of the plane of polarisation, and this rotation 
was reversed when he reversed the current. He next used a 
charged coil of wire in place of the magnet and got the same 
result, so that there was undoubtedly some definite connection 
between light and electricity. 

The next step was one that led to even more astonishing 
results, but these did not materialise till long after Faraday's 
time. He arranged a flame between the poles of a magnet and 
injected salts of sodium and lithium into it, and examined their 
spectra, but, alas ! he obtained no result. But that was not the 
fault of the experiment, nor was it a flaw in his hypothesis, for 
svhere Faraday failed, another physicist, called Zeernan, armed 
ater with much more powerful instruments, succeeded. We 


shall return to this later on, when we consider some of the great 
discoveries made in our own time. 

One final experiment before we leave Faraday and his 
wonderful works: " If iron/' he said, "is magnetic, why not 
other substances ?" If a bar of iron be swung between the 
poles of a magnet, it will place itself parallel to the lines of force 
i.e., in the line joining the north and south poles of the magnet. 
Faraday substituted a bar of heavy glass for the iron, and 
found, to his surprise, that it swung round until it came to rest 
at right angles to the lines of force, and so he concluded that all 
bodies may be divided into those that were para-magnetic 
i.e., those like iron which arranged themselves parallel with 
the lines of force, and dia-nmgnetic i.e., those which place 
themselves at right angles to these lines. Among the latter sub- 
stances he classed human flesh, and concluded that if a man were 
suspended horizontally between the poles of a huge magnet, 
he would come to rest at right angles to the lines of force. 

Although we have spent much time over Faraday and his 
discoveries, it is not too long when we take into account the 
enormously extended outlook he gave us into the wonders of 
Nature. We have learnt something of what he accomplished, 
but in telling of his work we have said little of the man himself 
since we left him as an assistant in Davy's laboratory* 

Just after he began his study of Ampere's work on the effect 
of an electric current on a magnetised needle, he took unto 
himself a wife. There was no honeymoon, for he simply 
brought his bride home to his rooms in the Royal Institution 
and went on with his work as if nothing had happened. He was 
soon afterwards made Director of the Laboratories, but his 
salary remained at 100 a year, with rooms, fuel and light. 
During the years that followed he made a good deal of money 
in fees for advice given on various scientific problems that 
were put to him for solution. He soon began to feel, however, 
that if he was to work out the puzzles that Nature was con- 
stantly offering him, he must decline all outside work and all 
hopes of making a fortune. We are the wealthier by Ms dis- 
coveries, which led to so many and great inventions, but he 
died a poor man. In 1838 his health gave way, and for two 


years he did practically nothing. In 1841 he went to Switzer- 
land, where the rest and clear mountain air seemed to work 
wonders. During this time societies, universities and govern- 
ments showered honours on him. He accepted these, but, at 
least at first, he declined a Civil List Pension of 300 a year, 
and it was only after urgent pressure that he was at length 
prevailed upon to accept it. His last years were peacefully 
spent with his wife in a house at Hampton Court, given to 
him by Queen Victoria. On one occasion, towards the end, 
an intimate friend visited him to ask after his health, when 
Faraday replied, *' Oh, just waiting." The messenger with 
the sable wings called for him on August 25, 1867, and he now 
lies at rest in Highgate Cemetery: 

" Was ever man so simple and so sage, so crowned and yet so careless of a 


Great Faraday, who made the world so wise, who loved the labour better 
than the wage." 

Spectrum Analysis* Before leaving the subject of physics a 
word or two must be said on a branch of it which, in recent years, 
has been enormously extended viz., spectrum analysis. When 
a narrow beam of white light strikes a prism it is resolved into a 
band of coloured rays (p. 50) , but we have already learnt that the 
visible spectrum is only a very small part of the entire spectrum 
(Fig. 52). In the visible part of the spectrum an English 
doctor, called Wollaston, in 1802, drew attention to certain 
dark streaks crossing the coloured bands, apparently indicating 
places where there was no light of any kind. More than ten 
years afterwards these dark streaks were rediscovered by 
Fraunhofer, an apprentice to an optician in Munich. Without 
knowing anything of Wollaston's work, Fraunhofer studied 
these dark lines, and gave them letter names. Fig. 76, A, gives 
the spectrum of sunlight, showing the position of the principal 
dark lines where Fraunhofer considered the sunlight was 
missing. After very careful examination he identified several 
hundreds of these lines, which are now always spoken of as 
' Fraunhofer 's lines." Light from the moon or from Venus 
jave the same spectrum as light from the sun, but that was 
>nly to be expected, seeing that these celestial bodies shine only 


by reflected light. But when he examined the great star, 
Aldebaran, he obtained a quite different spectrum (Fig. 76, 
C). At first Fraunhofer thought that it was the atmosphere 
surrounding our earth that blocked out some of the sun's rays, 
and naturally expected that light from a star would be affected 
in a similar manner. But seeing that this was not so, and 
knowing that Aldebaran shone by its own and not by reflected 
light, he concluded that sunlight must be different from star- 

The next step was taken in 1827 by the astronomer Sir 
John Herschel, and was based on the fact that if table-salt be 




O Y 

G B I V 

AaEc D Eb F G HH 






placed in the flame of a spirit-lamp the flame becomes bright 
yellow, and when this light is examined through a spectroscope 
it shows a bright yellow double streak at the point of Fraun- 
hofer's dark line, D, due to the metal sodium in the salt. 
Herschel suggested that if the bright lines given by luminous 
vapours were carefully mapped out, these, if constant, might be 
utilised for the identification of the same substances wherever 
they might occur. After a large number of metals had been 
examined in this way, HerscheFs idea was f ound to be entirely 
correct. No matter how many other substances are present, 
and no matter how minute the quantity of the material used, 
the spectrum lines belonging to that particular substance can 
always be identified. It may readily be imagined what a valu- 



able method this was likely to become, not only for identifying 
the faintest traces of known elements, but also for discovering 
new ones, and it was actually by means of the spectroscope that 
several new metals were found, such as caesium, rubidium, 
indium and gallium. The whole matter was very carefully 
studied and explained by two distinguished German scientists, 
Bunsen and Kirchhoff. 

The Bunsen Burner. Robert Bunsen was born in 1811, 
and, after holding teaching posts at Cassel, Gottingen and 
Marburg, became professor of chemistry at Heidelberg in 1862, 
where he remained until his death in 1899. His name is best 
known, perhaps, as the inventor of the "Bunsen 
burner/' without which no chemist can carry on his 
work. If a substance be burnt in an ordinary gas 
or oil flame there is always formed on it a deposit 
of soot due to the incomplete combustion of the gas 
or oil. Bunsen's invention consisted in placing a 
metal tube over an ordinary gas-jet, the bottom of 
the tube being pierced with holes to permit of air 
entering and mixing with the gas before being lit 
FIG. 77.- a t the top of the tube. In this way complete 
BinmER combustion was effected and a non-luminous flame 

resulted (Fig. 77). 

Gustav Robert Kirchhofi was born at Konigsberg in 1824, 
and became professor of physics first at Heidelberg and after- 
wards at Berlin, where he died in 1887. 

The Spectroscope. It was in 1859, while the two men were 
fellow-professors at Heidelberg, that they worked out the whole 
theory of spectrum analysis. The general principle of the 
spectroscope is shown at Fig. 78. T is a telescope (fixed 
colHmator) having at one end a narrow adjustable slit, S, which 
receives the beam of light, and at the other end a lens, L. A, 
A' are prisms by which the rays are refracted. This enables the 
spectrum to be spread out as a long band, and by shifting the 
position of the movable telescope, T', any part of the spectrum 
may be brought into the field of vision. Thus white light, for 
example, appears as a "continuous spectrum/' violet to the 
left and red to the right, when such white light is transmitted 


through the slit, S, and examined by the eyepiece, E, Sodium 
light, on the other hand, is monochromatic (see Fig. 76, S). 

Kirchhoff allowed a bright ray of sunlight to pass through 
the vapour of sodium, and found that the dark Fraunhof er line, 
D, appeared even darker than it was in the solar spectrum, and 
concluded that the sunbeam must have come through the 
vapour of sodium before reaching the earth, and that this 
vapour must be in the solar atmosphere, seeing that there is 
none of it in our own. In every case that he tried he got the 
same result, and so he was led to the general conclusion that any 
glowing or incandescent vapour absorbs from white light 
those rays that it itself gives out when glowing. Now since 
Fraunhofer had shown that the light from a star may differ 


from the light from the sun, the starlight must have passed 
through other incandescent vapours between the star and our 
eyes. It would thus be possible to determine what were the 
vapours that surrounded the star and the sun respectively. 

Sir William Herschel thought that the sun was a dark globe 
surrounded by a glowing atmosphere, but Kirchhoff 's discovery 
proved that, on the contrary, the sun was an intensely hot 
burning mass, surrounded by an envelope of incandescent 
vapour. By employing spectrum analysis we have learnt that, 
although many of our common elements (about forty) are pre- 
sent in the sun, such as hydrogen, oxygen, carbon, iron, copper 
and silver, yet there are many other equally common ones that 
do not seem to be there at all, such as sulphur, phosphorus, mer- 
cury, nitrogen and gold; they are probably there, however. 


Discovery of Helium. One of the most remarkable of the 
discoveries that have resulted from the use of the spectroscope 
is that made by the British astronomer, Sir Norman Lockyer, 
in 1868. He noticed a fine yellow line close to the double 
sodium pair, and since no other substance known to science 
gave this line, he concluded that he had found a new element. 
To it he gave the name of " helium/' the sun element, from 
the Greek word for the sun, helios. Twenty-seven years 
later another British scientist, the late Sir William Ramsay, 
while examining a rare Norwegian mineral called cl&veite, 
got from it a gas which gave the same spectrum line as helium, 
so that we have here an instance of an element identified in 
the sun, 93 millions of miles away, many years before it was 
discovered on the earth. Helium is what is called an " inert 
gas/' because it refuses to combine with any other element. 
It is non-inflammable and of low density, and hence is in- 
valuable for inflating balloons. During the early days of the 
Great War balloons for observational purposes were filled with 
hydrogen; but these, of course, could be instantly destroyed by 
a well-aimed shell. By and by great stores of helium were 
found both in the United States and in Canada, and, had the 
war not come to an end, there is no doubt that dirigibles would 
have always been filled with helium instead of with the in- 
flammable hydrogen. 

Another advance was made in the subject when the English 
astronomer, Huggins, began examining the spectra of stars 
and nebulae, but into his work we need not enter at present, as 
the results achieved belong rather to the later years of the 
nineteenth century. 

Here we must leave our study of physics for the time being 
and turn to the closely related science of chemistry, which we 
left in a rather backward state at the end of the seventeenth 
century. After that period it began to push on at a tremendous 
pace, until, today, it is perhaps the most forward of all the 



When we review the history of chemistry and try to realise 
the condition of the science in the seventeenth century, what 
amazes us most is the almost entire absence of any clear idea as 
to the essential difference between an element and a compound. 
As modern methods of separating one partner in a compound 
from another were entirely unknown, there were very many 
substances that were regarded as elements by the early chemists 
which were, later on, found to be compounds, such as lime, 
potash, soda and magnesia. 

The Balance in Chemical Experiments JOSEPH BLACK, 
The first chemist of the newer school was Joseph Black, the 
discoverer of the law of latent heat (p. 119). He was the first 
to introduce the general use of the balance into chemical mani- 
pulations. Before his time men were content with proving the 
presence or absence of a substance in any mixture that is 
to say, they were satisfied with qualitative tests; but Black 
insisted on the necessity for making quantitative measurements 
as well, and nowadays the " balance room/' fitted with instru- 
ments of great precision, is regarded as an essential part of 
every chemical laboratory. 

Discovery of Carbon Dioxide. One of Black's investiga- 
tions was concerned with the nature of limestone, in the course 
of which he made some extremely important discoveries. These 
may be repeated with the aid of very simple apparatus, costing 
only a few pence. 

Obtain some limestone and put a small piece into a test-tube 
containing some water. Nothing happens; but add a few drops 
of " spirit of salt," or what we call hydrochloric acid, and at 
once bubbles of gas are given off from the limestone, and this 
effervescence continues until all the limestone or acid has 
disappeared. Take another piece of limestone and heat 
it to redness; it becomes after a time a porous solid, which 
no longer effervesces with acid, but, on the other hand, if 
water be added it swells and becomes hot. From these very 
simple experiments we learn, first, that limestone is insoluble 
in pure water, but is acted upon by an acid with the loss 


of a gas, leaving something behind which is soluble in water. 
Second, we learn that heat alters the limestone in such a way 
that the substance left gives off no gas when treated with the 
acid, but changes into something else when water is added. 
We explain these facts nowadays thus : 

Limestone is a compound of carbon, oxygen and the metal 
calcium, namely calcium carbonate (CaC0 3 ), which is not 
soluble in pure water. When treated with hydrochloric acid 
(HC1), a compound of hydrogen and chlorine, the action results 
in the formation of three substances, water (H 2 0), the gas 
carbon dioxide (CO 2 ), found in the " choke-damp " formed 
in coal-mine explosions (p. 145), and a new substance, calcium 
chloride (CaCl 2 ), partly derived from the hydrochloric acid 
and partly from the limestone. Chemists write this change 

2 +CaCl a 

meaning that one molecular unit of limestone with two of hydro- 
chloric acid give one unit each of water, carbon dioxide and 
calcium chloride. This is what chemists call a " chemical 
equation/' and, as matter is neither created nor destroyed, 
everything on one side of the equation must appear on the other. 
If this equation be carefully studied it will be seen that on 
both sides there are three Os, two Hs, two Cls, one Ca and one C, 
although differently arranged. These symbols, 0, H, C, Ca 
and Cl stand for the chemists' atoms (see p. 184). 

When limestone is heated an invisible gas is given off, and 
there is left " quick " or active lime. Chemists represent what 
happens by this equation: CaC0 3 =CaO+C0 2 . Nothing has 
been added to the limestone, for heat, as we know from Count 
Rumford's work (p. 122), is not a substance, and hence does not 
appear in the equation. The gas given off is carbon dioxide, 
and the " quicklime " is a compound of calcium and oxygen 
only. When hydrochloric acid is added to quicklime there is 
no effervescence, because there is no carbon dioxide to be given 
off. When water is added to the " quicklime " (CaO), the 
latter at once begins to swell, heat is produced, and steam is 
given off, as may be seen where workmen are mixing lime and 


water to make mortar. What is happening in this case is 
not a breaking down or decomposition, but a construction or 
combination. Water and quicklime chemically unite to form 
"slaked" or satisfied lime, thus: 

CaO+H 2 O==CaH 2 O 2 or Ca(OH) 2 . 

If slaked lime be shaken up in water, some of it will dissolve, 
and the clear solution is sold in the shops as lime-water. 
Breathe through this lime-water and it becomes milky. Now 
when the white particles have settled to the bottom of the 
vessel as a sediment or "precipitate," pour off the excess 
fluid, dry the sediment, add a little dilute hydrochloric acid, 
and once more carbon dioxide is given off and the white powder 
disappears. In short, we have got back to where we started, 
for the sediment is calcium carbonate, the carbon dioxide 
having come from the breath and united with the lime. 

These mysterious changes, taking place in a substance sup- 
posed, in Black's time, to be an element, were very puzzling 
to the chemists of the eighteenth century, and Black set 
himself the task of solving the riddle, and attacked it in the 
following way. He weighed some 
pieces of limestone and placed them 
in a flask (Fig. 79, A) along with 
water, to which he added some 
acid, and by means of a bent tube, 

B, connected the flask with a re- 
ceiver, C, filled with water and 
standing mouth downwards, over a 

trough full of water, D. The gas _ 

to . , to FIG. 79. FORMATION OB- CARBON 

(CO a ) which collected in the vessel, DIOXIDE FROM LIMESTONE. 

C, was found to be the same in 

nature and amount as that got by heating chalk or limestone. 
Similar experiments with " magnesia alba " gave the same 

He next took some lime-water and bubbled the gas through 
it and obtained a white precipitate, which he proved to be identi- 
cal with limestone or chalk. Since the gas was originally 
" locked up " in the limestone, he called it " fixed air," and was 


thus able to show that limestone was not a simple substance 
but a compound of lime and " fixed air." When he inserted 
a lighted taper into a vessel with " fixed air " it went out. 
, He also collected the same gas from fermenting malt and certain 
mineral springs, as well as from air expired from the lungs. 

Robert Boyle had long previously obtained a blue pigment 
from a lichen (Lecanora), which is known as litmus, and which 
has the property of becoming red whenever it is brought in 
contact with an acid. By using a solution of litmus, a Swede, 
called Bergman, showed that "fixed air" was an acid, and 
finally, in 1779, the great French chemist, Lavoisier, proved that 
''fixed air 1 ' was a compound of carbon and oxygen, and 
renamed it carbonic acid. 

The Phlogiston Theory. During the seventeenth and 
most of the eighteenth centuries, nearly all scientific men 
believed in what is known as the " phlogiston theory " (p. 56) 
viz., that combustible bodies consisted of an invisible sub- 
stance called " phlogiston " and an ash or " calx/' and that the 
more phlogiston there was in any body the more combustible 
it was. It may be safely said that this chimera retarded 
progress in chemistry for at least a hundred years. It was 
quite on a par with that equally erroneous notion that heat 
was a "subtle fluid," disposed of by Rumford and Humphry 
Davy. But both these theories were generally accepted during 
the period we have mentioned. 

JOSEPH PRIESTLEY. At this time there lived a man who 
had been justly regarded as one of the founders of chemistry, 
but who might have earned even greater merit had he only 
recognised what a "will-o'-the-wisp" phlogiston really was. 
This was Joseph Priestley. Like so many other great men, he 
came of a humble stock, for his father was a cloth-dresser in a 
mill near Leeds, where Joseph was born in 1733. In his youth 
i he showed a considerable gift for learning languages, and so 
'his family chose for him the clerical profession. After his 
training for the ministry was completed, he became a non- 
conformist parson in a village in Suffolk. His congregation, 
however, did not approve of his theological principles, and so 
he left Suffolk for a corresponding^ post in Cheshire, finally 


becoming a teacher of languages at Warrington Academy. 
After six years in that town he returned to the ministry as the 
pastor of a chapel in Leeds. During all this time, in addition 
to carrying on his clerical and other duties, he had been devoting 
much of his energy to mastering what was then known of the 
sciences of physics and chemistry. It was while he was at 
Leeds that he published the first volume of his great work 
called "Experiments and Observations on Different Kinds 
of Air." 

Priestley was very skilful in inventing apparatus. Recog- 
nising that bladders were unsuitable articles in which to collect 
gases, he brought into use glass cylinders, which he filled with 
water or mercury, afterwards to be displaced by the gas to be 
examined. This was called a " pneumatic trough/' used by 
Black, but also by Priestley in 1772, five years before Black 
published his work on " fixed air/' Carbon dioxide was known 
to be present in large quantities in certain mineral springs, and 
Priestley pointed out that the exhilarating effect of these 
waters was due to the presence in them of this gas which could 
be made to dissolve in ordinary water under pressure. When 
small quantities of potash and soda were added, a refreshing 
drink was obtained, and this was the starting-point of the 
great industry of mineral water manufacture so familiar to us 

Priestley remained in Leeds for six years, but left that city 
to take up the post of librarian to Sir William Petty, afterwards 
the first Marquis of Lansdowne. This nobleman had been one 
of the Secretaries of State, but had recently resigned owing 
to differences of opinion with the other members of the Govern- 
ment over the policy to be followed with regard to the American 
colonies. The Marquis was deeply interested in scientific 
matters, and Priestley, now with a comfortable home and an 
adequate salary, had abundant opportunity for carrying out 
research work, in which his patron aided him in every way 
possible. Among other advantages derived from his friendship 
with the Marquis was the opportunity of meeting other famous 
chemists, especially Lavoisier, whom they visited during a 
tour on the Continent. 


Discovery of Oxygen. Let us now turn to the discovery 
with which Priestley's name is almost always associated viz., 
the discovery of oxygen gas. 

Exactly one hundred years previously, the young Oxford 
doctor, Mayow, had shown that only a portion of any measured 
quantity of atmospheric air was used up by a burning candle 
or a living animal enclosed in a vessel containing air, and thus 
realised that this proportion of the atmosphere was essential, 
both to combustion and to respiration. Mayow called it 
" spiritus nitro-sereus." It is true he did not actually prepare 
it, though had he lived there can be little doubt that he would 
have done so. 

Priestley, on the other hand, has the credit of actually 
isolating the gas. He used a pneumatic trough (Fig. 80, A) 

filled with mercury, in 
which he arranged a 
vessel, B, also filled with 
that metal. In the bulb 
of the retort, C, he 
placed a quantity of red 
oxide of mercury, and 
then concentrated solar 
rays on it by means of 
the lens, D. Presently 
the mercury oxide be- 
gan to give off a gas 

OXYGEN GAS. wlucl1 collected in B, 

while glittering drops of 

mercury made their appearance in the retort. He thus proved 
that red oxide of mercury was a compound of the metal 
mercury and a gas, which, instead of extinguishing a lighted 
taper, made it burn more brightly. Finding also that a 
mouse enclosed in a bell-jar of the gas became more lively, 
he concluded that it could not be injurious to health, and 
therefore decided to breathe the gas himself. He felt so elated 
from its effects that he said, " Some day soon this will be a 
fashionable luxury/ 1 

Although Priestley (in 1774) had thus really isolated oxygen, 


he missed the most important consequence of his discovery 
by his persistent belief in the phlogiston theory. He even 
went so far as to call the new gas " dephlogisticated air** 
that is to say, air that would not burn because it had been 
deprived of its phlogiston. 

Priestley remained with the Marquis of Lansdowne for 
seven years, but afterwards returned once more to clerical 
work, this time in Birmingham. While engaged in these 
chemical investigations, he found time to write books and 
pamphlets on theological subjects, and on the relations of 
Church and State, and made himself much disliked by his out- 
spoken views. When he got the length of expressing publicly 
his sympathy with the French Revolutionists, matters came to 
a crisis. He was so unpopular in the midland capital that his 
house was burnt to the ground by an infuriated mob, and he and 
his family barely escaped lynching. He thereupon removed 
to London, but even there his unpopularity pursued him, and 
at length he crossed the seas to the New World, where he died 
in 1804, 

SCHEELE. While Priestley was experimenting with red 
oxide of mercury and investigating oxygen gas, a Swedish 
apothecary, called Scheele, was working on the same subject. 
Towards the end of the eighteenth century the means of com- 
munication between workers in science was not what it is now. 
Today, if a man makes a great discovery, it is known all over the 
world in a few hours, even in a few minutes if it be " broad- 
casted." The newspapers tell their readers about it the next 
morning, and the story is given in full to some learned society 
within a few days, or at most a few weeks. It was not so in 
1774 when Priestley discovered oxygen. Scheele had been ex- 
perimenting on gases for some years, quite unaware of Priest- 
ley's doings, as Priestley was ignorant of Schede's work. Both 
published their results in book form, Priestley in 1774 and 
Scheele in 1777, so that Priestley has the credit of being first 
in the field. It is true that his book came out three years 
before Scheele's "Chemical Treatise on Air and Fire/' but 
there is every reason to believe that Scheele had isolated 
oxygen at least two years before Priestley did. The whole story 


of Scheele's life and labours was, indeed, very imperfectly 
known until the Swedish Academy gave it to the world in 1892. 

Carl Wtthekn Scheele, who was born in 1742, was one of a 
family of eleven, and his opportunities for acquiring knowledge 
were few. At the age of fourteen he became apprentice to an 
apothecary in Gothenburg, and taught himself chemistry with 
the aid of the books and apparatus he found in his master's 
shop. Indeed, the story of his early life much resembles that 
of Humphry Davy, who was also an apprentice to a pharmacist, 
and who also taught himself chemistry very much in the same 
way (p. 140). Davy became a very distinguished man both in 
science and in society, but Scheele never rose above his humble 
beginnings as a country apothecary. There was no Count 
Rumford in Sweden to offer him a post in a great scientific 
institution, where his genius might have had full opportunity of 
developing, and where wealth might have provided him with 
the apparatus he required. 

When he had reached the age of twenty-six he removed to 
the capital, but still as an assistant in a ' ' chemist's " shop. His 
first work was the production of an inflammable gas by immers- 
ing iron filings in dilute acid. At that time iron was supposed to 
be a compound of a calx and a little phlogiston, according to 
those who held the phlogiston theory, of whom Scheele was one. 
He thought that in the inflammable gas he had produced from 
the mixture of iron filings and acid he had at last captured 
the mysterious phlogiston, and called it " phlogiston elasti- 
cum " ; what he had really discovered, although he did not 
know it, was hydrogen. 

Scheele's Discovery of Oxygen. In 1770 Scheele went to 
Upsala, where he made the acquaintance of Bergman, who was 
at that time professor of chemistry in the university there. It 
was he who showed that Black's " fixed air " was an acid, but 
otherwise he was not one of the leading lights in the science. 
It was at Bergman's instance that Scheele began to examine 
what the Roman historian, Pliny, had called ll black magnesia." 
From this substance, which is really an oxide of the metal 
manganese, Scheele obtained oxygen. There are several com- 
pounds of manganese and oxygen, and related to them is a salt 


called permanganate of potash which yields up its surplus 
oxygen very readily in the presence of putrefying organic 
matter, removing its offensive odour almost at once. When 
dissolved in water, in which it is very soluble, it forms a very 
beautiful violet solution, often used for staining floors. Sodium 
permanganate is now sold as a disinfectant under the name of 
" Condy's Fluid." Scheele obtained oxygen from several other 
substances, and, incidentally, isolated not only manganese but 
also the metal barium and the gas chlorine, afterwards identified 
as an element by Sir Humphry Davy. 

The only honour Scheele ever received was that of being 
made a member of the Royal Academy of Sciences, an honour 
he richly deserved. The last few years of his life were spent 
in the little country town of Koping, where he died in 1786, 
at the early age of forty-four. 

HENRY CAVENDISH. We must now turn our attention to 
a person of a very different character, a member of one of 
England's best- known families, the House of Devonshire. The 
Hon. Henry Cavendish is celebrated in science not only as a 
chemist but also as a physicist, for by an ingenious apparatus 
he was able to measure the density of the earth, and that to 
within a fraction of the result obtained by modern physicists, 
using his method, but with much more delicate instruments. 
We shall return to this subject later when we speak of the dis- 
coveries made in our own time (p. 348). 

Notwithstanding the fact that he came of such notable 
parentage and made so great a name for himself in science, it 
is remarkable how little we know about him. He was extra- 
ordinarily reserved and retiring, mixing with very few people. 
He never spoke to his servants if he could possibly help it, and 
was even in the habit of leaving a note on the hall table saying 
what he wished for dinner ! He was a very wealthy man, and 
although he gave liberally when he was asked to do so, he 
never tried to find out deserving cases of poverty for himself. 
When he did give, his gift was seldom accompanied by the little 
kindly sympathy that would have doubled its value. A friend 
of his once said, " Cavendish did some good in a very ungracious 
manner." His love of solitude and his abhorrence of fuss and 


ceremony was almost a mania with him, and nothing illustrates 
this hermit-like characteristic better than the story told by Ms 
doctor, who records that "one evening, when Cavendish was very 
ill, he dismissed his valet, saying that he did not wish to be 
disturbed as he had something very important to think about. 
It was something he and we have to think about only once, 
for he died before morning ! 

The problems at which he used to work were, when finished, 
as often as not, put away in pigeonholes and never looked at 
again, an idiosyncrasy he shares with Sir Isaac Newton, who 
had a similar habit of hiding the results of his researches. 

Cavendish was born in 1731, and educated at a private 
school, and afterwards at the University of Cambridge, where 
there is now a memorial to him in the form of the Cavendish 
Physical Laboratories. Little is known as to how he spent his 
time between the date of his leaving Cambridge in 1753 until 
1766, when he published a paper on gases in the Transactions of 
the Royal Society, and for many years afterwards he appears to 
have been engaged on research on similar subjects. It was not 
until 1784, however, that he made his great discovery of the 
composition of water, which he announced in his work called 
" Experiments on Air. 

The Composition of Water. Cavendish had already studied 
the properties of "inflammable air" or hydrogen, and made 
use of the knowledge he had gained in his later experiments. 
He obtained hydrogen by pouring dilute sulphuric acid over 
zinc, and collecting the gas given off by means of a pneumatic 
trough (p. 169). When he mixed a measured volume of this 
gas with about two and a half times its volume of ordinary 
air and set fire to the mixture, an explosion took place, and 
the wall of the container became dim with dew, which he 
identified as water. 

In order to determine the exact proportions of the two 
gases he invented an instrument which he called a eudio- 
meter (Fig. 81). He prepared a mixture of 195 volumes of 
oxygen and- 370 volumes of hydrogen in a bell-jar, and, from 
this reserve, filled the pear-shaped bulb, A, of the eudiometer, 
from which he had previously extracted all the ordinary air by 


an air-pump. The tap, B, was then closed, and an electric 
spark was sent through the terminals, C C. In his own words 
he said the gas "lost its elasticity and became liquid water." 
This on condensation gave room for more of the gaseous mixture, 
and that was again fired and changed into water, and so on, 
for six times in succession. The water so formed was produced 
by the chemical union of 2 volumes of hydrogen with one 
volume of oxygen, but it was distinctly acid. This 
impurity he identified as nitric acid caused by the 
accidental presence of nitrogen . Nitrogen was the 
gas left over from ordinary air after the oxygen 
had been removed by combustion. It had been 
noted by Mayow in his experiments (p. 58), and 
had been called "mephitic air/ J in 1772, by a 
chemist named Daniel Rutherford. The word is 
of Greek origin, and means a " poisonous exhala- 
tion," but we speak of it now as nitrogen. It is 
called in Cavendish's book " phlogisticated air," 
and a careful analysis proved to him that the 
nitrogen was not pure, but that a small propor- 
tion, which he estimated at one-hundred and 
twentieth of its volume, consisted of something 
else, the nature of which he was unable to deter- 
mine. His final analysis gave the composition of 
atmospheric air as 20*833 per cent, by volume of 
oxygen and 79-167 per cent, of nitrogen, the latter 
figure including the unknown "impurity." More than a cen- 
tury afterwards Lord Rayleigh and Professor Sir William 
Ramsay discovered its nature (chiefly argon), and incidentally 
added several new elements to the growing list. The proportion 
of these together came to about 0-94 per cent, of any given 
volume of nitrogen, so that Cavendish, with his much cruder 
apparatus, made a very close approximation to the estimate 
arrived at today with the most delicate instruments modern 
scientific workshops can produce. 

During the latter half of the seventeenth century and the 
whole of the eighteenth, facts of chemistry were thus rapidly 
accumulating, but isolated facts, as we have already said, do 


FIG. 81. 


not make a science, they must be woven into a connected 
whole. Moreover, the nomenclature of chemistry was largely 
in a state of chaos; indeed, the whole subject awaited a master 
hand to mould it into shape, and do for it what Newton did for 
astronomy and Hutton did for geology. This master was 
forthcoming in the person of Antoine Laurent Lavoisier. 

LAVOISIER. Lavoisier was born in 1743, and was the son 
of a lawyer, and he himself was educated with the view of 
following the same profession. But the lure of science was too 
great, and by the time he had reached his twentieth year he 
had forsaken the study of law for that of chemistry. Experi- 
mental science in general and chemistry in particular required 
apparatus, and apparatus, especially in those days, was costly, 
so that, although his family were by no means poorly off, 
Lavoisier had to earn an income somehow. At first he was 
Director of the Government powder works, but shortly after- 
wards he became a member of a sort of syndicate, called the 
" Ferme Generate," which had for its purpose the coEection of 
taxes on foreign and imported goods, paying the Government 
a certain sum annually for the privilege of doing so. The 
Government was thus relieved of the labour of gathering these 
taxes, and was assured of a definite income from the Ferme, 
while that body benefited by all profits it made on the trans- 
action. That the profits were considerable is shown by the 
fact that most of the men of the syndicate lived in great luxury 
and, by their extravagance, roused popular indignation against 
them. There is no reason to believe that Lavoisier acted as 
a tax collector in any but an honest and kindly way, and 
whatever he made out of his post he devoted to the purchase of 
apparatus and the equipment of his laboratory. Nevertheless, 
he did not escape the hatred of the taxpayers, a hatred that 
was ultimately to lead to his death. 

When he was twenty-eight years old Lavoisier married a 
girl of fourteen, who was the daughter of one of his colleagues 
in the Ferme. Mere child as she was she showed herself to be 
possessed of a remarkable brain, and used it to assist her 
husband by translating chemical treatises in other languages 
into French, and in sketching and engraving apparatus to illus- 


trate Ms writings. Many years afterwards she became the 
wife of Count Rumford, but that second marriage, by all 
accounts, was not a success. 

One of the last of Lavoisier's public services was connected 
with the Commission that devised the metric system of weights 
and measures, now universally used in science. He also took 
a keen interest in the affairs of the Academy of Sciences, which 
corresponded to our Royal Society of London, but, in 1793, 
the National Convention suppressed all the learned societies, 
and all the members of the Ferme Generale were arrested and 
imprisoned shortly afterwards. A special appeal was made on 
behalf of Lavoisier as being a distinguished scientist who had, 
by his work, not only done good service to the State but had 
conferred great glory on France by his discoveries, but the 
appeal was of no avail. One of his judges at the trial brutally 
remarked, " Scientists ! The Republic has no use for scientists/* 
A few hours afterwards he was hurried away to his execution. 
The great mathematician, Lagrange, said of this judicial murder, 
" A moment was all that was necessary in which to strike off 
this head, and probably a hundred years will not be sufficient 
to produce another like it." 

Lavoisier's Use of the Balance. We have already seen 
that one of Black's chief merits was his constant use of the 
balance, by means of which he obtained accurate quantitative 
results where his predecessors had been content with qualitative 
ones only (p. 165). Lavoisier fully recognised the value of 
the balance, and in all his researches brought it into play 
wherever he could. He was the first to introduce the chemical 
equation to represent every chemical change, and to insist that 
the two sides of the equation must exactly balance each other. 

It was an ancient belief among chemists that water could be 
transformed into earth, but this Lavoisier proved to be perfectly 
erroneous in the following way. He used a vessel of known 
weight, called from its shape a " pelican/' a flask with a long 
neck bent over in a loop so as to re-enter the flask at a lower 
level. Into this he introduced a measured quantity of pure 
water, sealed the flask, and applied gentle heat. The water 
vapour produced passed over into the narrower part of the 



vessel, where it condensed and trickled back into the bulb. 
After three months of continuous heating he evaporated off 
all the water and obtained a small sediment, which he collected 
and weighed. The empty " pelican " was also weighed, and it 
was found that the weight of the sediment was exactly equal 
to the weight the vessel had lost. The only possible conclusion 
was that the water, unchanged in itself, had dissolved some part 
of the glass itself. 

Lavoisier's Completion of Priestley's Work. During their 
visit to the Continent, Priestley and his patron, the Marquis of 
Lansdowne, made Lavoisier's acquaintance, and Priestley told 
him how he had obtained " dephlogisticated air" by heating 
red oxide of mercury. Lavoisier at once set to work on the 
problem of determining the nature of the gas that had been 

isolated by Priestley, repeating his 
experiments, but in the reverse way 
{.&, by causing "dephlogisticated 
air " to unite with mercury, under 
quantitative conditions (Fig, 82). 

He placed 4 ounces of mercury in 
a retort, R. The retort had a bent 

FIG, 82.-OxiDATioN OF neck open^g fc^o a ben-jar, G, stand- 

MERCURY. . rr\ -r-r 

ing over mercury in a trougn, 1 . He 

calculated the volume of the air in the retort and bell-jar, and 
found it to be 50 cubic inches. He then heated the retort 
by means of the furnace F, and presently saw a reddish scum 
appearing on the surface of the mercury. He kept the experi- 
ment going for twelve days, until the formation of the red scum 
had ceased, and then proceeded to measure and weigh the pro- 
ducts, He first of all found that there were now only 42 cubic 
inches of gas in the retort and bell-jar, while the red scum 
weighed 45 grains. Placing these 45 grains in another retort 
and heating it, as Priestley had done, he obtained 4iJ grains 
of mercury and 8 cubic inches of gas. The 42 cubic inches 
of gas left over from the first experiment he found to consist 
of " mephitic air/' or, as he called it, " azote " (from the Greek 
words " a" not, and " zoe" life), since it could not support 
life, while the 8 cubic inches of gas obtained from the second 


experiment was " dephlogisticated air/' or what Lavoisier at 
first called "vital air/' since life was impossible in its absence. 
When the 42 cubic inches of (t azote " and the 8 cubic inches 
of " vital air " were mixed together he obtained the 50 cubic 
inches of atmospheric air with which he started. The cycle 
was thus complete. 

The New Chemistry. After numerous experiments of a 
similar nature, he put forward four general principles or laws 
as the basis of his new chemistry. The first was that substances 
burn only when " vital air " is present. His second proposition 
was that all non-metals, such as phosphorus, sulphur, or carbon, 
when burnt in vital air, give rise to acids, phosphoric, sulphuric, 
or carbonic, and for that reason he renamed his vital air 
"oxygen/' from the Greek words: oxus, tart or acid; and 
gennao, to produce. In this second generalisation he was 'over- 
hasty, ^ for hydrochloric acid, HC1, contains no oxygen. 
Lavoisier's third principle was that a metal when burnt in air 
formed a calx by combining with oxygen, becoming heavier in 
the process. Thus, when the metal magnesium is ignited in 
oxygen it produces a white powder, magnesia, which is heavier 
than the original magnesium by the weight of the oxygen it has 
fixed from the air. Lastly, he affirmed that there was no 
such thing as phlogiston, and that every case of combustion 
was due to the union, under proper conditions, of the com- 
bustible body with the oxygen of the atmosphere. The 
phlogistonists did not give in without a straggle, but Caven- 
dish's synthesis of water from hydrogen and oxygen drove 
the last nail into the coffin of phlogiston, and the new century 
knew the name no more, save in an historical relation. 

In the year 1789, just five years before chemistry lost her 
new prophet, Lavoisier published his famous "Elementary 
Treatise on Chemistry," in which he expounded his views on 
the science in general, views that are now universally accepted, 
This book resulted in a complete change in the nomenclature of 
the science. " Inflammable air " became hydrogen the water 
producer; "phlogisticated air " became nitrogen, since its union 
with oxygen the acid producer gave nitric acid. Com- 
pounds of oxygen with metals were known as oxides which, 


when united with acids, formed>alts. Moreover, since oxygen 
united with sulphur, phosphorus, and so on in varying pro- 
portions, the terminations of the names of these acids were 
altered to suit. Thus sulphur with relatively more oxygen gave 
sulphuric acid, and with less gave sulphurous acid; similarly 
for nitric and nitrous acids and phosphoric and phosphorous 
acids. The salts of these acids were respectively sulphates and 
, sulphites, nitrates and nitrites, etc. In short, the whole vocabu- 
lary of chemistry took the form that we are so familiar with in 
the textbooks of the present day. 

It is seldom, indeed, in the history of science, that so great 
a revolution has been brought about by the publication of one 
comparatively small book. Even in the almost unexplored 
domain of organic chemistry Lavoisier pointed out the way 
to be followed; for he realised that it was possible here also to 
work backwards, so to speak, and by carefully weighing the 
products obtained from the combustion of substances like 
alcohol or sugar, to determine how much carbon, hydrogen 
and oxygen was in the original substance, and to reconstruct 
it at least, on paper. 

The early years of the nineteenth century, therefore, saw 
an entire change in the outlook on chemistry. Lavoisier's 
work had almost at once caused the evaporation of the mists 
that had for so long clouded the whole subject of chemical 
combination, and now it was possible to follow all the changes 
in the test-tube or retort with a clear vision of what was taking 
place. Phlogiston was seen to have been merely a will-o'-the- 
wisp, leading the student into a morass in which he was doomed 
to flounder hopelessly. Alas ! Lavoisier did not live long enough 
to dispel other misleading ideas in chemistry, but he paved the 
way for the triumph of the atomic theory. 

The Atomic Theory of Matter. Away back in the dawn of 
science the old Greek philosopher, Democritus, imagined the 
universe to be composed of infinitesimally small particles or 
atoms, differing in size, weight and shape, and held that these 
atoms were eternal, invisible and as the word signifies 
indivisible. This was not the only view held amongst the 
Greeks, for Xenophanes, who lived about the time of Pythagoras, 


rather believed that matter was continuous, with no gaps 
between the particles. We need not discuss these ancient 
guesses at the structure of matter, nor the interesting specula- 
tions on the subject made by the Roman poet Lucretius and 
others, but come at once to the views on atoms put forward 
just after Lavoisier's death. 

The Law of Definite Proportions. Before considering 
more modern views on atoms, we must draw attention to four 
laws of chemistry that were established about this time by four 
different men, working independently in four different countries. 
The first of these men was Joseph Louis Proust, a French 
apothecary, who was born in 1755, and who, after a brilliant 
university career in Paris, became professor of chemistry in 
Madrid. Here he carried out most of his research work, until 
the Peninsular War brought about the destruction of his 
laboratory and all that it contained. His labours in the 
Spanish capital led to the establishment of the first of the four 
chemical laws to which we have referred. 

Berthollet had argued that chemical compounds varied in 
character according to circumstances, and that the force in- 
ducing the union of elements acted somewhat in the same way 
as gravity that is to say, that the affinity between any two 
bodies depended on their respective masses. It was thought, 
for instance, that a metal could unite with oxygen in gradually 
increasing proportions, and that the composition of the resulting 
substance was not immutable. Proust denied this, and said 
that an oxide of iron or of any other metal always had the same 
composition whether it occurred free in nature or was manu- 
factured in the laboratory, and, further, that when the metal 
or non-metal united with oxygen to form two or more com- 
pounds, there was always a definite gap between any two suc- 
cessive oxides. To t^ke a modern exam^e, the oxides of nitro- 
gen, nitrous oxide (N 2 0), nitric oxide (NO), nitrogen trioxide 
(l! 2 O 3 ), nitrogen dioxide (N 2 O 4 or N0 2 ), and nitrogen pentoxide 
(N 2 5 ), Proust insisted that there could not be any compound 
intermediate between any two successive members of the series. 
This is known as the law of multiple proportions. 

The next step, and the most fundamental, which follows 


logically from Proust's law, was taken by John Dalton. Dalton's 
simple but revolutionary conception of combining weight was 
as follows: When one element united with another in more 
than one proportion, the weight ratios were some simple multiple 
of the lowest proportion. The combining weight of an element 
thus represented the weight of the atom of that element, relative 
to that of the atoms of other elements. Looking at the list of 
compounds of nitrogen and oxygen given on p. 181, it will be 
seen that two atoms of nitrogen unite with i, 2, 3, 4, or 5 atoms 
of oxygen, but never with a fraction of a whole number. The 
law of multiple proportions is at once intelligible in terms of 
Dalton's atoms. 

The Law of Combination of Gases. The third law was 
established by a French chemist, Gay-Lussac, who was born 
in 1778 in Auvergne. After a somewhat exciting boyhood, 
spent during the troublous days of the French Revolution, 
Gay-Lussac became a teacher in the Paris Polytechnique. 
In a balloon ascent he collected samples of the atmosphere at 
various heights, up to nearly five miles, and found that the 
relative composition of dry air always remained constant. The 
law with which his name is primarily associated is that known 
as the law of volumes. Following on Cavendish's demonstration 
that two volumes of hydrogen always unite with one volume of 
oxygen in producing water, he went on to prove that two volumes 
of carbon monoxide (now written CO) unite with one volume of 
oxygen to form two volumes of carbon dioxide, now expressed 
2CO+0 2 = : 2C02; that one volume of hydrogen and one of 
chlorine produce two volumes of hydrogen chloride,* and so on. 
In 1808, he announced his law viz., that when gases combine 
they do so in volumes bearing a simple ratio to each other, and 
the product formed, if a gas, has a volume bearing a simple 
ratio to the original volumes. 

Equal Volumes of Gases at the same Temperature and 
Pressure contain Equal Numbers of Molecules. The fourth law 
was one established in 1811 by an Italian who rejoiced in a name 
of no less than ten words, but who, for short, is always known 

* This is now written H 2 +C1 2 = 2HCI (hydrogen chloride or hydrochloric 
acid gas), though not understood at the time. 


as Avogadro. He was born in Turin in 1776, and for several 
years practised as a lawyer, although, at the same time, he was 
much interested in experimental science. In 1820 he became 
professor of physics in the University of Turin, and held that 
post until 1850, save during a period when, for political reasons, 
the chair was in abeyance . Into his other work we need not enter, 
but will confine ourselves to the law always known by his name. 
The chemical formula for water is, as we have seen, H 2 ; 
why should it not be H0 } ? Because that would mean splitting 
an atom of oxygen, for represents the smallest possible quan- 
tity of that element, just as H 2 represents the smallest possible 
quantity of water. To this unit of water Avogadro gave the 
name of "molecule."* A molecule of an element may consist of 
one or more atoms; thus a molecule of mercury is Hg, of oxygen 

H 2 H 2 2 H 2 H 2 


is O 2 , of ozone O 3 . Fig. 83 represents the union of hydrogen 
and oxygen to form water. If n represents any number, in 
the present instance 4, then since a molecule of hydrogen is 
composed of two atoms and a molecule of oxygen is also com- 
posed of two atoms, 4^ atoms of hydrogen will unite with 2,n 
atoms of oxygen to form zn molecules of water. Now 
Avogadro's law states that at the same temperature and pres- 
sure equal volumes of these gases contain equal numbers, of 
molecules. In the figure each square represents one volume, 
and there are two volumes of hydrogen, each containing four 
molecules of paired atoms, and one volume of oxygen containing 
four molecules of paired atoms, giving two volumes of water 
vapour, each containing four molecules of triple atoms. But 
this was not understood in 1811; until 1858 the formula for 
water was HO where stood for a weight of 8. 

* This was not understood at the time, and the lack of distinction between 
atom and molecule, for the smallest units of elements, caused dire confusion 
for nearly half a century, 


Dalton's Atomic Theory . Let us now consider the general 
atomic theory put forward by Dalton. John Dalton came of 
Quaker stock, settled near Cockermouth in Cumberland. He 
was born in 1766, and was the son of a weaver, who also farmed 
a small croft. At the age of fifteen he joined his brother in 
keeping a boarding school at Kendal, but the venture was not 
a financial success, so in 1793 he removed to Manchester to 
take up the duties of science lecturer in New College. After six 
years of this work he gave it up, and supported himself by 
private tuition. In later life he was granted a Civil List 
pension, and so was able to devote himself entirely to his 
favourite study, chemistry. He never married, but boarded 
with a clerical friend in Manchester for the last twenty-five 
years of his life. He died in 1844 at the age of seventy-eight. 

As we have just seen, on the foundations of the Laws of 
Definite and Multiple Proportions, he based his "Atomic 
Theory." He started by assuming that every chemical 
element was composed of excessively minute particles or 
"atoms/' possessing certain definite characters which never 
change, no matter how they may be compounded with other 
atoms. These atoms were, he held, incapable of division: 
" Thou knowest thou canst not cut an atom/ 1 he said. Every 
atom of an element is identical with every atom of the same 
element, but each separate element is composed of atoms of 
different weight. What was the weight of an atom ? Exceed- 
ingly small, of course, but it must have some weight. Having 
no means of estimating such infinitesimal quantities, he took 
as unit weight the element hydrogen, the lightest known, while 
the combining weights of the atoms of other elements were 
regarded as multiples of unity i.e., atomic weights. Since 
i Dalton erroneously supposed, on Avogadro's basis, that equal 
'.volumes of different gases contained .the same number of atoms, 
if the weight of a volume of chlorine, combining with one volume 
of hydrogen, was thirty-five times that of the volume of hydro- 
gen, then the atomic weight of chlorine was thirty-five. Simi- 
larly he determined the atomic weight of oxygen to be eight as 
compared with hydrogen. In modern chemistry we call these 
'' equivalent weights/' and proper atomic weights could not be 


deduced till the true significance of Avogadro's hypothesis was 
grasped in 1858 (Cannizzaro). So water for many years was 
given the erroneous formula HO. 

Hydrogen as the Primary Element Prout's Hypothesis. 
From the earliest times of which we have any record philo- 
sophers have always been striving to arrive at simple laws 
that would explain and govern many apparently disconnected 
facts, so when Dalton's atomic theory was made public, it was 
only natural that some chemists should begin to speculate 
as to the possibility of all the elements being made up of 
atoms of hydrogen. The first to put forward this view was 
William Prout, who was born at Horton in Gloucestershire in 
1785, and ultimately graduated as a doctor of medicine of 
Edinburgh. After a study of Dalton's theory, he published 
a paper in 1815 in which he gave it as his opinion that the 
atomic weights of the various elements were all multiples of the 
lightest of them viz., hydrogen which he called the " pro- 
tyle," from the Greek words protos, first, and hide, stuff. 
Thus all matter would be built up of this single primordial stuff, 
though the genesis of the elements remained unexplained. At 
first this rather alluring hypothesis " caught on," but when more 
accurate estimates of atomic weights came to be made, it was seen 
that they were by no means always whole numbers, and so the 
idea was gradually abandoned. Itwas revived in a newf orm many 
years afterwards, and as we shall find Prout was not far out. 

Estimation of Atomic and Molecular Weights Chemical 
Symbols. Very important work in connection with atoms and 
molecules was carried out in the early years of the nineteenth 
century by the Swedish chemist Berzelius. He was the son of 
a schoolmaster in East Gothland, and was born in 1779. After 
a somewhat broken educational career, he took his medical 
degree at the University of Upsala in 1801, and ultimately 
became professor at the Military Academy at Stockholm. As 
this was the period when Dalton was publishing his atomic 
theory, Berzelius took a keen interest in the subject, and, 
with great labour, estimated the atomic and molecular weights 
of over 2,000 substances, and, in the course of doing so, dis- 
covered several new elements. We have a constant reminder 


of him every time we write a chemical formula, for it is to him 
that we owe the chemical symbols for the elements in use today, 
so far as these elements were known in his day. 

Sir Humphry Davy's Work on Chemistry. Sir Humphry 
Davy was not only a great physicist but also a great chemist. 
Indeed, his first work in science was connected with the nature 
of the gases used in the Bristol Pneumatic Institute (p. 140), 
and his experiments included the inhalation of various gases, 
whose effects on the human body were quite unknown. In 
the course of his investigations he very often risked his life, 
for to breathe carbon dioxide, hydrogen, nitrogen, marsh gas 
and so on, was, to say the least of it, a very risky proceeding. 
Soon after his appointment to the Royal Institution, he began 
giving a course of lectures on agricultural chemistry, and con- 
tinued doing so for several years. These lectures were pub- 
lished in 1813, and formed the most important textbook on 
the subject for the next fifty years. When he turned his atten- 
tion to electricity he very soon, as we have seen, applied the 
method of electrolysis to the decomposition of substances like 
potash and soda, and isolated the metals potassium and sodium, 
so that these two alkalis turned out to be compounds and not 
elements. He next investigated what were called the " alkaline 
earths," baryta, strontia and lime, and found that they were 
oxides of three new metals, barium, strontium and calcium. 

The Nature of Chlorine. Scheele had found that when 
sulphuric acid, or "oil of vitriol," was poured on a mixture 
of black oxide of manganese and common salt, a greenish gas 
was given off. This gas when united with hydrogen formed 
what was called " spirit of salt " or muriatic acid, from muria, 
the Latin name for brine (it is formed when vitriol acts on salt 
alone and is now called hydrochloric acid) . Lavoisier thought 
the green gas must be an acid, and therefore must contain 
oxygen. Scheele also tried to analyse it, but, beyond noticing 
that the gas bleached vegetable colouring matters, he could make 
nothing of it. Davy tried heating it with aU sorts of substances, 
but never obtained any oxide, so he at last came to the conclusion 
that it must be an element, and gave it the name of " chlorine/" 
from the Greek word chloros, green the colour of the gas. 


Davy thus added very materially to our knowledge of 
chemistry, for we must not forget that his invention of the 
safety lamp was based not only on his knowledge of physics 
but also on his knowledge of the chemical compounds, marsh 
gas, carbon dioxide and carbon monoxide, which played so 
important a part in his own invention. 

Other great names in chemistry now begin to appear on the 
horizon, but as these are associated with the tremendous 
developments of modern times, we may leave them over for 
the moment and turn to the last of the five great sciences, 


Increase in Knowledge of Plants and Animals. Ever since 
the days when Columbus, Vasco da Gama, Magellan and Cabot 
sailed into the unknown to find new lands, there had been hosts 
of travellers who had explored the newly discovered countries, 
or less ambitious persons who had wandered over regions nearer 
home, seeking to acquire knowledge of Nature and her produc- 
tions. Many of these were students of natural history, who 
brought back with them new and strange plants and animals 
they had met with in their travels. 

While the old Greek biologist, Theophrastus, knew at 
least 500 different kinds of plants, the herbalists of the 
fifteenth century described over 2,000, and these they arranged 
either alphabetically or in groups, according to what they 
called their " vertues " i.e., their uses to man in medicine or 
the arts. Still new forms flowed in, until in the seventeenth 
century over 10,000 kinds of plants were known. At the 
present moment we know of at least 400,000 species of plants, 
while the numbers in the animal world are greater still. The 
late Sir Arthur Shipley gives a total for the animal kingdom 
of 597,000, of which the insects alone account for no less than 

The problem before the botanist and zoologist was obviously 
to arrange all this mass of material into some sort of scheme, 
so Aat, when a new plant or animal was discovered, a place for 
it might be found. 


Nomenclature of Plants and Animals. In the early days a 
plant or an animal was not known, as it is now, by two names, 
but by short sentences describing some marked peculiarity it 
possessed. One of the first to recognise the clumsiness of 
this method was a German herbalist, called Kaspar Bauhin, 
who lived in the early years of the seventeenth century. He 
attempted to give plants two names only, one the generic or 
surname, the other the specific or Christian name, and, as is 
the custom in a town directory, the surname came first not 
John Smith, but Smith, John. Thus a buttercup was called 
Ranunculus acris, not acris Ranunculus, and other kinds of 
buttercups, differing in certain details, received specific names 
indicating the peculiarity in question e.g., hairy, hirsutus] 
aquatic, aquaticus; bulbous, bulbosus, etc. The names chosen 
were most commonly Latin, because Latin was the language 
used by the learned in those days, and every botanist, no matter 
what his nationality, could recognise the names of the plants 
in a language which was common to all nations. 

Different genera were next seen to show still broader like- 
nesses, and were grouped together in families, and these into 
yet larger divisions, such as those we found Ray created viz., 
dicotyledons and monocotyledons. Precisely the same system 
of naming was adopted for animals. Of course, the early 
naturalists did not rename all plants and animals on this plan, 
but they made a beginning at it, and since then it has always 
been called the binomial system of nomenclature, and is now 
universally adopted. 

CARL LINNJEUS. Nearly a century after Bauhin's time, 
a much more famous man pressed on the reform, and carried it 
out in its entirety for both plants and animals; this was Carl 
Linnaeus. Before attempting to estimate the services Linnaeus 
rendered to biology, it will be well to know something of the life 
of this remarkable man. 

Linnaeus was born in 1707 at Rishult in Sweden, where his 
father was a pastor. It is not generally known that " Linnaeus " 
was not his real name, but Ignomarsen; but owing to his 
father's house being overshadowed by a grove of lime or linden 
trees, the family came to be called Lindelius, which became 


modified into Linnaeus, and by that name he is always known. 
As is not unusually the case, the father's views as to his son's 
future did not find favour with Carl, for while the pastor 
wished his son to follow the clerical profession, Carl spent his 
time collecting natural history specimens. His school reports 
were so bad that his father gave up the idea of the ministry as a 
career for him, and arranged to apprentice him to a shoemaker 1 
Fortunately the local doctor took an interest in the lad, and 
at his instigation Carl was sent to the University of Lund to 
study medicine. From there he went to Upsala, where he 
became assistant to the aged professor of botany, Rudbeck, 
after whom is named the small yellow sunflower so often 
cultivated in our gardens. 

In 1732, when he was still an undergraduate, he went on 
an exploring and collecting expedition to Lapland, and on his 
return tried to support himself by teaching science. In this, 
however, he was unsuccessful, not because he had no pupils, 
but because he was, by regulations, disqualified from receiving 
payment for giving instruction without possessing a university 
degree. Without money he could not obtain a degree, and 
without a degree he could not even earn a living, so there seemed 
to be nothing left but to return to the cobbler's shop. But then 
a young lady came to the rescue. She was the daughter of a 
wealthy physician named Moraeus, but although the two young 
people had settled their future to their own satisfaction, the 
doctor intervened and refused his consent to their betrothal, 
let alone marriage, until Linnaeus had taken his degree and set 
up in practice for himself. Then the girl showed her quality 
and her faith in her lover, for she handed over to Carl all her 
own money, which, added to his savings, enabled him to go to 
Holland, where, in due course, he graduated as doctor of medicine 
in the University of Harderwyk, at the age of twenty-eight. 

But his period of service for his Swedish Rachel was not yet 
complete. Instead of hurrying home to put up his brass plate 
he went to Leyden, then a great seat of learning, and there he 
made the acquaintance of Boerhaave, the famous professor of 
medicine, who obtained for Linnaeus the ppst of physician to the 
burgomaster of Amsterdam, who was himself a horticulturist of 


some repute. The burgomaster sent Linnseus to England to 
obtain specimens for the Botanic Gardens at Amsterdam, and 
during his visit he met some of the leading scientific Englishmen 
of the day. It was about this time that he published the first of the 
long series of books for which he afterwards became so famous. 

More than three years had passed since he had bidden 
farewell to his fiancee, and now that he was known as a 
scientific man of high standing, his future father-in-law with- 
drew all opposition to the wedding, and so at last he won his 
reward, and the lady was repaid for her trust and loyalty. After 
a short time spent in Stockholm, Linnaeus was appointed to 
succeed Rudbeck in the University of Upsala, at the age of 
thirty-four, and was now free to devote himself to the science 
in which he had made his name known aU over Europe. In 
1749 he had over 100 students in his class, but that figure was 
soon greatly exceeded, for, mainly owing to the reputation he 
had acquired as a teacher, the number of students in the 
university rose from 500 to 1,500. In Upsala he remained for 
the rest of his life, dying there in 1778 at the age of seventy-one. 

The " Systema Naturae " and Binomial Nomenclature. Let 
us now see in what respects Linnaeus advanced the science of 
biology. When he went to Leyden he took with him a work 
on which he had been engaged for some years, and which he 
called the " Systema Naturae," or general scheme of Nature. 
When first published in 1755 it comprised only twelve folio 
pages of printed matter, merely an outline of his ideas on how 
to arrange plants, animals and minerals. No less than twelve 
editions of the " Systema " were brought out during his life- 
time, each one an extension of its predecessor. Zoologists, 
as a rule, take the tenth edition, of 1758, as their starting-point, 
while botanists prefer another work called the "Species 
Plantarum," published in 1753, as their guide to the naming of 
plants. The important point to remember is that, both in 
these and in other works which we need not mention, Linnaeus 
brought into universal use the binomial method of naming 
plants and animals that had been suggested by Kaspar Bauhin 
more than a century before. In the mode of describing plants 
and animals Linnaeus also made considerable improvements. 


Instead of the long-winded descriptions of plants that the 
herbalists indulged in, he gave the essential characters in one 
short sentence in which there was not even a verb. Thus the 
wild rose, Rosa canina, was described as " the common rose 
of the woods with a flesh-coloured, sweet-smelling flower. 3 ' 

The Conception of Species. Another feature of Linnaeus's 
teaching was the emphasis he laid on the idea of a " species/' 
but here he showed himself far behind John Ray. Before 
Ray's time the word "species" was used in the vaguest 
possible way, just as our modern newspapers talk of the 
" human species " or the " orchid species/' although there is 
only one species of human being and something like 8,000 
species of orchids. Ray, on the other hand, considered a species 
as all the individuals that have arisen from similar parents 
and that give rise to similar offspring. He noticed, however, 
that species were not always strictly constant; every now and 
then seedlings might grow into plants that differed considerably 
from their parents, and so he recognised the importance of varia- 
tion. In the earlier editions of his work Linnseus insisted that 
species were constant and immutable. He held that in the 
beginning, when the world was first stocked with plants and 
animals, some 6,000 years ago, there was one pair of each kind 
of organism to start with, and that all the lions, tigers, dogs, 
daisies, roses, lilies and so on were the direct descendants of 
the original pairs of each that had been created. Indeed, he 
expressed this view quite dogmatically in a well-known and 
often quoted sentence: " There are never any new species; there 
are just as many species now as there were forms created by 
the Infinite Being in the beginning/' Later on, he could not 
help seeing how common and how widespread variation really 
was, and in later editions of the " Systema " he does not express 
himself quite so positively on the matter. 

Linnaeus never studied the minute structure of either the 
plant or of the animal in the way that Grew and Malpighi did; 
he knew little or nothing of how the machine worked, in other 
words, of the subject of physiology really the most important 
part of biology. Again, holding the views that he did on 
the constancy of species, he could not recognise the existence 



of any genealogical relationship between them. What was 
left ? Classification, but not the kind of classification we aim 
at nowadays, where we strive to group animals and plants in 
such a way as to show their probable ancestral origins, but a 
classification which was merely a catalogue arranged in divisions 
and subdivisions to facilitate the naming of some unknown 

The Sexual System in Plant Classification. One of 
Linnaeus's greatest achievements was his so-called "Sexual 
System " of classifying plants. As a matter of fact, the system 
was not sexual at all; sex did not actually come into the 
question, although the apparent organs of sex did. It may be 
as well to justify this statement. On examining any common 
flower, such as a buttercup (Fig. 84, A), it is easy to recognise 

, outside, or lower down, five 
greenish leaves called sepals, 
S; these are followed by five 
yellow petals, P, and these again 
by numerous slender threads 
with club-shaped heads, the 
stamens, A, containing a yellow 
powder, the male fertilising 
material or pollen; and finally, 

in the centre, a number of ovoid green bodies called carpels, 
G, which contain ovules and are, therefore, regarded as female 
organs. In another flower (Fig. 84, B) e.g., of Enchanter's 
Nightshade the same four kinds of structures may be dis- 
tinguished, but there are only two of each, the pair of carpels 
(G) being joined together and sunk below the level of the other 
parts, though continued upwards as a slender stalk or style, 
ending in a knob-shaped head, called the stigma, G'. 

In his "Sexual System" of classifying plants, Linnaeus 
made great use of these parts of the flower, dividing them into 
groups, distinguished, first, by the number of stamens they had; 
and these groups he again split up according to the number of 
carpels, or rather styles, they possessed. Such a classification 
would be equivalent to that exhibited by a city directory where 
all the Smiths, Wilsons, or Robinsons are grouped together, 




each group subdivided according to the Christian names, quite 
irrespective of their real family relationships. Let us give 
him the credit, however, of realising that this plant directory 
was not a true system of classification, but only a makeshift, 
useful for cataloguing temporarily the thousands of plants 
that were already known, and the thousands more that were 
being discovered year by year. He himself warned his readers 
that his scheme was meant for convenience only, and to form 
a guide to the rapid naming of any particular plant. In later 
years he made an attempt in his " Philosophia Botanica" 
at drawing up a natural system based on real likeness in struc- 
ture, taking all features into account, but this work he never 

It will thus be seen that, although Linnaeus published an 
enormous amount of material and carried with Mm crowds of 
enthusiastic followers, it cannot be honestly said that he made 
any discoveries in biology comparable with those made by 
Davy or Faraday in physics, or by Lavoisier in chemistry. 
Indeed, one distinguished modern botanist, Professor Sachs^of 
Wiirzburg, said of him that he showed " an utter incapacity 
for careful investigation of any subject at all difficult to 
observe/' In these circumstances, it may, perhaps, be asked, 
if he contributed so little to the advancement of the science why 
mention him at all ? For the same reason that Ptolemy's 
name is so important in the history of astronomy. He, too, 
produced a system, not of plants and animals, but of the 
universe, which was accepted by the learned of all nations for 
over a thousand years, and yet which was utterly erroneous. 
So, too, Linnseus put forward a classification of plants that 
became extinct before a hundred years had passed. But just 
as Ptolemy's mistaken ideas led Copernicus to write the 
"Revolution of the Celestial Bodies/ 1 so Linnseus's quite 
unnatural system was indirectly the cause of the birth of the- 
natural system put forward by the French school of botanists 
some fifty years after his death. 

There is no risk of his name being forgotten, for it lives in 
the title of the third great scientific dub of natural historians 
in Britain, the " Linnean Society/' at a meeting of which, a 



century afterwards, the doctrine of the constancy of species on 
which Linnaeus pinned his faith was completely overthrown 
by the greatest of all biologists Charles Darwin. 

General Aspects of Biology. We may regard the science of 
biology from several aspects. First of all, we may consider 
plants and animals from the purely structural point of view, 
their external form or morphology, their internal constitution or 
anatomy with its corollary, histology i.e., the study of tissues 
and on the knowledge we thus acquire we formulate schemes of 
classification. If our classification is to be in the true sense 
natural, it must take into account all parts of the organism, 
not merely one selected feature. Further, in formulating 
such a classification we must not neglect to take into account 
the representatives of the plant and animal worlds long 
since extinct, and now available only in the form of fossilised 

No matter how detailed our knowledge of these subjects 
may be, we have not thereby acquired any adequate con- 
ception of the science of biology, which, as the name indicates, 
is the study of living things, while morphology, anatomy, and 
histology might more appropriately be termed necrology, a 
study of corpses, from the Greek word nekros, dead. It is not 
enough to analyse a complex machine at rest, we must also 
study it in motion, if we are to gain any real acquaintance with 
the uses of its various parts. This department of biological 
knowledge is termed physiology, the study of functions. A 
very casual glance at the living organism shows us that the 
functions of plant and animal alike may be grouped under 
three headings: first, those concerned with feeding or nutrition; 
second, those concerned with multiplication or reproduction; 
and, third, those concerned with response to stimuli or sensi- 

There is yet another aspect of biological study, perhaps the 
most difficult of all viz., that of origins. This is called 
phylogeny, from two Greek words meaning "lineage of the 
tribe/' With these three departments of biology morphology, 
physiology and phylogeny biologists in Linnaeus's time were 
most unequally acquainted. Linnaeus himself had a good 


knowledge of external morphology, but he does not seem to 
have used the microscope at all. He was in no sense a physio- 
logist, and for him, with his belief in special creation and the 
constancy of species, phylogeny did not exist. In the years 
following Linnaeus, the biological outlook broadened; physiology 
became of first-rate importance, and the dogma of the con- 
stancy of species began to give way to the doctrine of evolution. 

CUVIER. One of the first to break new ground, so far at 
least as the animal kingdom was concerned, was Georges Leo- 
pold Cuvier. His family were Protestant refugees from the 
region of the Jura, where Georges was born in 1769. He was 
destined for the clerical profession, but his keen interest in 
natural history soon led to his forsaking the study of theology. 
In 1788, owing to his family's reduced circumstances, he had 
to take up teaching, and became tutor to the son of a nobleman 
near Caen, where he remained for six years. Here he spent 
much of his time dissecting marine animals. Accounts of his 
doings reached the ears of the leading zoologists in Paris, and, 
as a consequence, he was invited to become an assistant in the 
Jardin des Plantes. He accepted the post, and ultimately 
became professor of comparative anatomy there ! He after- 
wards rose to high rank in the State under Napoleon, and was 
created an Officer of the Legion of Honour in 1826, and a baron 
in 1831, He did not live long to enjoy his rank, for he died of 
paralysis in 1832. 

Comparative Anatomy and Palaeontology. When we turn 
to Cuvier's work in biology, we find him in some respects 
quite as retrograde as Linnseus. In spite of the work of Redi- 
Leeuwenhoek, and others (p. 66), he believed firmly in spon- 
taneous generation of life from inorganic or dead material. 
He thought that the fertilised egg contained within it the whole 
adult organism in miniature, and that development was merely 
an unfolding or expansion of its parts; also he was as staunch 
an upholder of the dogma of the constancy of species as Linnseus 
was himself. But he certainly opened up a new field that 
Linnaeus had never explored, that of comparative anatomy. 
He published an important textbook on that subject, in which 
he dealt with the minute structure of both higher and lower 


animals. Not content with studying living forms, he worked 
out the nature of the animals whose fossil bones were to he 
found plentifully in the neighbourhood of Paris, and from 
these fragments reconstructed the organisms as they might 
have appeared in past ages. 

The Classification of Animals. Cuvier 's greatest work was 
" The Animal Kingdom arranged according to its Organisation/' 
which was published in 1816. In this magnificent treatise he 
laid down the general principle that animals are built in one 
or other of four types: first, the Vertebrata, including all those 
that possess a bony skeleton with a backbone; second, the 
Mollusca, such as snails, limpets, mussels, etc., soft-bodied 
animals without an internal skeleton, but often with shells; 
third, Articulata, jointed animals like lobsters, worms and 
insects; and fourth, Radiata, or animals whose bodies were 
radially symmetrical, like starfish, sea anemones, polyps and 
such like. 

Correlation and Adaptation. Another of his doctrines was 
the "correlation of parts," meaning that every organism is 
a co-ordinate whole; if any part be changed in any way, every 
other part must change also. If, for instance, an animal 
possesses a stomach adapted to digest raw flesh, its teeth are 
suited to tear the flesh to pieces, its claws to seize and hold it, 
its senses to recognise the presence of its prey and its limbs to 
overtake it. Such animals he called Carnivora, but the various 
subtypes of carnivora have special peculiarities of their own, 
special kinds of teeth, claws, etc., fitted for distinct kinds of 
prey, and created to meet these ends. 

SAINT-HILAIRE. Geoffrey Saint-Hilaire was born in 1772^ 
and died in 1844. He was at first a colleague of Cuvier at the 
Jardin des Plantes, but later became professor of zoology at 
the Museum of Natural History. In opposition to Cuvier, 
Saint-Hilaire insisted on the principle of homology i.e., he 
held that the organs of animals were not specially created to 
fulfil a definite purpose, but that when conditions were altered 
an organ carrying out another duty could be changed to suit 
the new circumstances. The controversy between these two 
distinguished zoologists marks the first rumblings of the storm 


that broke out a few years later, when the whole subject was 
reopened by the publication of Lamarck's great work on 
" Animals without Vertebrae." 

De Jussieu's Classification of Plants. Returning to the 
plant world, we find that Linnseus's system of classification had 
been adopted not only in his own country, Sweden, but also 
in Germany and England; but perhaps the esteem, not to say 
reverence, in which that great naturalist was held, had as much 
to do with the general acceptance of his views as the inherent 
merits of the system itself. At all events, the " SexualSystem " 
never caught on in France, where Ray's ideas were preferred. 
Ray's system was cordially received by Antoine L. de Jussieu, 
who made it the basis of his own scheme published in 1789. 
Antoine was the nephew of Bernard de Jussieu, the custodian 
of the Royal Gardens at Versailles, who had laid out the beds 
in the gardens in accordance with the views as to relationship 
expressed by Ray. He never published anything himself, so 
that we may assume that his nephew's books etabody any 
original views he held. 

De Jussieu's treatise was called the " Genera of Plants/ 1 and 
in it we find an extension of Ray's " Method/' which he adopts 
and expands. He 
divides plants in- 
to Dicotyledons, 
and Acotyledons, 
the last group 
including all the B 

lower plants, such ' 


as ferns, mosses, Cf EPIGYNY. 

fungi, seaweeds, 

etc. In his classification of flowering plants de Jussieu makes a 
great point of the position of the stamens and petals in their rela- 
tion to the carpels, speaking of them as ' 'hypogynous " when they 
arise below the level of the carpels, " perigynous " when they 
spring out round them, and " epigynous " when the carpels are 
entirely beneath the stamens and petals (Fig. 85). This was a 
most unfortunate feature to select, for if he had only traced the 


development of the flowers of some of the Saxifrages, he would 
have seen that in this family there were cases where the flower 
started by being hypogynous, passed through a perigynous 
condition and ended in being epigynous. It was the old mistake 
that Linnaeus made of basing a classification on one character 

PYRAME DE CANDOLLE. The next important step was taken 
by two members of a very distinguished family of botanists, 
that of de Candolle. The first of these was Augustin Pyrame 
de Candolle, who was born in Geneva in 1778. After studying 
in the university in that city, he went to Paris, where he made 
the acquaintance of Cuvier, Saint-Hilaire and the other 
naturalists who had their headquarters at the Jardin des Plantes 
and the Museum of Natural History. After spending ten years 
in Paris he became professor of botany at Montpellier, but after 
another decade he returned to Geneva to hold a similar post in 
its university. While he was at Montpellier he published a 
very important little book, which in its general character recalls 
Lavoisier's " Elementary Treatise on Chemistry," because it 
laid down the fundamental principles of the subject as they 
shaped themselves in de Candolle's mind. He called the book 
an " Elementary Theory of Botany." It was published in 
1813, and was the first textbook on the science. 

The Doctrine of Symmetry. The chief thesis de Candolle 
sets out to establish is that the foundation of classification is 
form and structure, and that physiology is of 
no value as a guide to relationship, indeed, that 
it is often misleading. Heathen expounds what 
he calls the symmetry of organs. To under- 
stand this doctrine requires the study and com- 
parison of a large number of forms, from which 
FIG. 86. SNAP- it can be seen that the fundamental symmetry 
DRAGON. becomes obscured by three causes: degenera- 
tion of parts, abortion of parts and adherence of parts of one 
kind to parts of another. Thus the flower of the snapdragon 
(Fig. 86) has only four stamens and two carpels, while, in 
accordance with the doctrine of symmetry, it ought to have 
five stamens and five carpels. One stamen, X, is entirely 


absent, but in figwort there is a scale where the missing 
stamen ought to be i.e., the fifth stamen has degenerated 
in the figwort and aborted in the snapdragon. 

The adherence of parts is a frequent feature in flowers. In 
the flower of the orchid, for example (Fig. 87), which is built on 
the trimerous plan i.e., with parts in threes 
there is only one stamen, and that is fused to 
the style. " The whole art of classification," 
de CandoHe says, "consists in discovering the 
plan of symmetry." 

Notwithstanding that he held such views, ^ 

it is strange that he was at the same time ORCHID. 
an upholder of the dogma of the constancy 
of species. If a flower possesses four functional stamens 
and one degenerate one, when, according to the doctrine of 
symmetry, it ought to have five perfect stamens, it must 
either have been created with this useless vestige of a stamen, 
and then there could not have been any degeneration, or it 
must have been derived from some ancestral type with five 
functional stamens, and then there could be no constancy. 

The Prodromus. The classification of plants put forward 
by de Candolle, though an improvement on that of de Jussieu, 
was still very imperfect from our modern standpoint, but it 
served as a protest against the purely artificial schemes that 
followed the lines laid down by Linnaeus, and it also formed the 
basis of the arrangement adopted in some form or another in 
many of the floras of today. 

In addition to his efforts at establishing a " Natural Classi- 
fication," as it is called, de Candolle began an immense work, 
" The Prodromus," or Outline of the Plant Kingdom, continued 
by his son Alphonse, with the aid of several other botanists. 
Begun in 1824, it was not completed until 1873, when it com- 
prised seventeen volumes of over 12,000 pages. It professed 
to be an account of every known flowering plant, arranged 
according to de Candolle's scheme; but as our views on the 
relationships of the higher plants underwent enormous changes 
after the middle of the nineteenth century, it is now regarded 
as a work of reference only. 


Bentham's " Genera Plantarum." One of the botanists 
who aided Alphonse de Candolle in completing his father's 
work was George Bentham. He was born in 1800 at Stoke 
near Portsmouth, and in his earlier years studied law and 
philosophy. In 1830 he made the acquaintance of Alphonse 
de Candolle, and began contributing to the great Prodromus, 
giving up law as a profession altogether. He travelled ex- 
tensively, making collections which he ultimately presented 
to the Herbarium at Kew. Almost any day he was to be 
found there, working quietly and systematically at the de- 
scription of flowering plants. He became very friendly with 
Joseph Dalton Hooker, the son of Sir William Hooker, who 
was at that time Director of Kew Gardens, and was induced 
to take part in the preparation of a series of floras of the British 
Colonies, which had for some time been contemplated by the 
Kew authorities. While engaged on these labours, Bentham 
constantly found himself in difficulties with the limits of genera 
in the vegetable kingdom, and he soon realised that what was 
most needed at the moment was a clear statement as to the 
precise characters that constituted a genus. This led him to 
plan out his greatest work, the " Genera Plantarum/' in the 
compilation of which he had the assistance of young Hooker. 
The general idea was to settle the limits of genera first and 
allow the grouping of these to suggest themselves naturally, 
instead of plotting out a scheme in the first instance and forcing 
the genera into it. The " Genera Plantarum " appeared at in- 
tervals between 1865 and 1883, and is still regarded as the 
standard work on the subject. 

We need not discuss the classification scheme that resulted, 
beyond saying that it was a modification and extension of that 
of de Candolle; what is more important for us to notice is that 
throughout the entire work there is no reference to the lower 
plants; it follows the old plan of dealing with flowering plants 
only, and ignores the very existence of the host of non-flowering 
forms that, even in the first half of the nineteenth century, 
were beginning to receive attention from botanists. But a 
more remarkable point still is that no reference is made to the 
doctrine of evolution which, after the middle of the century, 


began to leaven all the writings of biologists. Bentham was 
a firm believer in the dogma of the constancy of species, and 
although, towards the end of his life, he felt himself driven to 
accept the new views, his conversion came too late to make it 
possible to alter the plan of the " Genera." 

The British Flora. To Bentham we also owe the " Hand- 
book of the British Flora/' a book which, to this day, is the 
standard guide to the naming of our native plants. It is a 
work that is highly valued by every field botanist and con- 
stantly referred to, and yet Bentham tells us that he " amused 
himself by writing it before breakfast." 

Plant Anatomy. While de Jussieu, de Candolle, Bentham, 
and Hooker were thus elaborating classifications of flowering 
plants on what was called the "Natural System," there were 
many who devoted themselves to the microscopic study of 
plants, and who attempted to work out the structure of the 
various tissues and the way in which they were combined in the 
organism. The microscope was as yet a very inferior instru- 
ment when compared with what is now in use; indeed, one 
marvels how these early microscopists managed to make out so 
much with an instrument the modern student would despise. 

It will be remembered that Robert Hooke, in 1665 (p. 64), 
had opened up the way to an understanding of the architecture 
of the organism by his discovery of the " cell "; but he and his 
successors, like Grew and Malpighi, confined their attention to 
the very obvious walls of the cells, and never enquired into 
the nature of the cell contents, which were lumped together as 
" sap," " vital juice," or " slime." It was not until 1812 that 
the anatomy of plants was placed on a new footing by the work 
of a German botanist called Moldenhawer, who was born in 
Hamburg in 1766, and ultimately became professor of botany 
at Kiel. He was the first to introduce the method of macera- 
tion or separation of the tissue elements by their prolonged 
immersion in water to which some acid had been added. By 
this means he was able to show that each cell and fibre had 
a wall of its own, and that they were not merely cavities in a 
homogeneous matrix, like bubbles in a foam. These cells were 
united in various ways to form different kinds of tissue, storage, 


conductive, protective and so on, while these tissues in turn 
were grouped together in layers and strands to form roots, 
stems and leaves, etc. 

Animal Anatomy. On the zoological side also there were 
many who were using the microscope to discover the nature of 
the building materials of the animal body, and one of the most 
prominent of these investigators was Bichat, a French surgeon, 
who was born in 1771. Hitherto the anatomist had been con- 
tent to analyse the animal body into organs, stomach, intestines, 
lung, heart, kidney and so on but Bichat went deeper and 
analysed these organs in turn into tissues muscular, nervous, 
glandular, etc. The next step was a reconsideration of the 
elements out of which the tissues were made, and the nature of 
the substances that were found in them. 

ROBERT BROWN. The first important advance on these 
lines was made by Robert Brown, the son of a Scottish clergy- 
man in Montrose. He was destined for the medical profession, 
but devoted his leisure moments to the study of the plants of 
his native land. After a period spent as a medical officer in 
the army, he went on a surveying expedition to the Antipodes, 
where he spent four years studying and collecting examples of 
the then almost unknown flora of Australia and Tasmania. 
On his return to England he became librarian to the Linnean 
Society, where he had access to Linnseus's great herbarium. 
During the next five years he published several papers on the 
structure of the various groups characteristic of the Australian 
vegetation, and compared the flora with that of South Africa, 
South America and other regions of the southern hemisphere, 
and so laid the foundations of botanical geography, a subject 
greatly extended by Sir Joseph Hooker many years later. 

The Structure of the Seed. In these monographs Brown 
announced many discoveries in plant morphology and anatomy 
which are now commonplaces in all the textbooks. For 
example, he worked out the structure of the ovule and the seed, 
and showed that those of the pine, larch, fir and their allies 
differed from those of the ordinary flowering plant in being 
exposed on open scales or carpels, while those of the flowering 
plant were enclosed or hidden. Hence he was led to separate 


plants of the pine alliance from all others, as "Gymno- 
sperms" i.e., naked seeded from ordinary flowering plants, 
which he named " Angiosperms " i.e., hidden seeded. He 
also studied the structure of the fertilising dust or pollen, and 
showed how pollen grains germinated on the stigma of the carpel 
and formed pollen tubes, which bored their way into the ovule 
and so brought about fertilisation. 

Brownian Movement. While investigating the passage of 
the contents of pollen tube into the ovule, Brown noted that 
the granules in the tube were in constant tremulous motion, 
a phenomenon common to all minute particles floating in a 
liquid. Brown offered no explanation of this peculiar motion, 
known since his time as " Brownian Movement/' but in recent 
years interpreted as due to the bombardment of such particles 
by the molecules of the medium in which they are suspended. 

Discovery of the Cell Nucleus. In one of his papers dealing 
with the curious flowers of orchids he made a discovery which 
led to the" foundation of a new section of his- 
tologyCytology, or the study of the structure 
of the cell. In the superficial layer, or epidermis, 
of the leaf he noticed that each of the cells of 
which it was composed contained a definite 
granule or "nucleus," and further research 
convinced him that this body was present in all 
living cells (Fig. 88). We now know that the 
nucleus is a structure of supreme importance 
both in the plant and in the animal; that it 
governs the growth and division of the cell, and 
is the bearer of hereditary characters from 
parent to offspring. Papers and books innumer- 
able have been written on the nucleus and VEGETABLE CELL - 
its behaviour during cell growth and division, 
enough indeed to form a library in themselves; 
and some universities have gone the length 

- i _e t - j- /- j i 

oi creating special professorships of Cytology; 

and all this has developed out of Brown's quite incidental 

observations on the leaves of some Orchidaceae. 

There is scarcely a single department of botany on which 

FIG. 88. 

lus; P d ' P lastid >" 

v * vacuole. 


Brown did not leave Ms mark. The distinguished American 
botanist, Asa Gray, said of him, " Perhaps no naturalist ever 
taught so much in writing so little, or made so few statements 
that had to be recalled or even recast/' and the great naturalist' 
and traveller, Von Humboldt, conferred on Brown a title so 
well merited that it was unanimously confirmed by all his 
fellow-botanists: "Easily the first of all botanists, the glory 
and ornament of Britain/' 

Discovery of Protoplasm. After the discovery of the 
nucleus by Robert Brown, several workers began to study the 
other contents of the cell, and it was not long before the general 
" sap," " slime/' or " vital juice " that the earlier anatomists 
had noted as more or less filling, the cell, and in which the 
nucleus lay, was found invariably to contain nitrogen. The 
same chemical discovery was also made in animal cells, 
where a nucleus was also found to be universaEy present. In 
the animal the cell-wall was observed to be not nearly so 
prominent a feature as in the plant, so that it began to dawn 
on men's minds that perhaps the contents were the most im- 
portant things in the cell, and that the wall was of secondary 
value. To the nitrogenous contents of the cell the French 
anatomist Dujardin gave the name " sarcode " or (e flesh/ 1 
and showed it to be the real basis of the cell, the wall being 
merely an excretion from it. At length, in 1844, the German 
botanist, von Mohl, announced that, in his opinion, the 
" slime " of the plant cell and the " sarcode " of the animal cell 
were identical, and proposed the name " protoplasm " or " the 
first formed substance " to cover both, a name now universally 
accepted. Several years later the distinguished zoologist, 
Huxley, defined protoplasm as " the physical basis of Hfe/' 
for it is to its activities that all the wonderful phenomena are 
due that we sum up under the term " life/' It has been ana- 
lysed hundreds of times, and although we are able to say that 
it consists of at least half a dozen chemical elements, combined 
in an almost infinite variety of ways, and linked together some- 
how to make a complex and everchanging whole, we are as yet 
entirely ignorant how a particle of this mysterious substance is 
able to manifest all the remarkable changes which we associate 


with the functions of nutrition, sensitivity and reproduction. 
The protoplasm of a plant ovum presents the same chemical 
characters as those of an animal ovum: the two protoplasms 
look the same under the highest powers of our best micro- 
scopes, and yet one gives rise to a forest tree, the other, perhaps, 
to a human being. Here we might truly say we meet with the 
supreme "riddle of the universe." When, if ever, we learn 
exactly how the constituent units of the protoplasmic frame- 
work are linked together, and what are the multifarious changes 
taking place in it, then, perhaps, we may learn how to make 
it in the laboratory. Even so we would not have created life. 

The Cell Theory. Another important question that 
occupied the attention of both zoologists and botanists of 
the middle years of last century was the mode of origin of 
the varied types of cell found in the plant and animal body, 
and the result of their investigations was the establishment 
of a doctrine known as the " Cell Theory/* This doctrine, 
which was worked out chiefly by two German biologists 
Schleiden, a botanist, and Schwann, a zoologist might be ex- 
pressed by saying that all tissue elements, no matter what then- 
ultimate forms and functions may be, are derived from primary 
cells, uniform in appearance and structure, and all of these in 
turn from a fertilised egg cell. Every cell arises from a pre- 
existing cell; " Omnis cellula e cellula " is a formula which 
reminds one of Harvey's famous aphorism, " Omne vivum ex 
ovo," every living thing comes from an egg. 

Foundation of Cryptogamic Botany. When we were con- 
sidering the growth of the idea of a natural classification of 
plants, we had occasion to note that all the lower forms of 
vegetable life were practically ignored, for during the closing 
years of the eighteenth and the early years of the nineteenth 
centuries very little was known either of the structure or the 
life-histories of the hosts of organisms represented by ferns, 
mosses, seaweeds and fungi. By the middle of the nineteenth 
century, however, several botanists began to enquire into the 
nature of the Cryptogamia, or flowerless plants, and a con- 
siderable amount of information was slowly accumulated with 
regard to them. As, however, our knowledge of these organisms 


is a product of the work done during the latter half of the 
century, we may leave that subject over for the present. 

Foundation of Palaeophytology. In another department of 
botany also considerable progress was made during the early 
years of the nineteenth century, due largely to the labours of 
Adolphe Brongniart, who, in 1828, did for the vegetable kingdom 
what Cuvier had done for the animal. He examined with great 
care and skill the fossilised remains of the vegetation of past 
ages, accounts of which he published in a long series of mono- 
graphs entitled " A History of Fossil Plants." Brongniart was 
born in 1801, and became professor at the Jardin des Plantes, 
He divided the whole geological series of strata into four epochs, 
each characterised by the dominance of certain types, but 
although more recent research has shown that his generalisa- 
tions cannot be maintained, his detailed work is still regarded 
as classical. 

Pioneer Work in Photosynthesis. It will be remembered 
that, in 1774, Priestley wrote a book called " Experiments and 
Observations on Different Kinds of Air " (p. 169). In this he 
speaks of " the restoration of air, in which a candle has burnt 
out, by vegetation. " He describes how a friend of his told 
him that while he was waiting for a boat to convey him to the 
Continent, he observed, at his inn at Harwich, a horse trough 
which the landlord refused to have cleaned out, because he 
found that the water remained longer sweet when the sides and 
bottom of the trough were " covered by a green substance 
which is known to be of a vegetable nature." After describing 
the " spontaneous emission of dephlogisticated air (oxygen) 
from water containing a vegetative green matter," he says he 
never found the emission took place save when the water was 
exposed to light. 

Ingenhousz on the Gaseous Exchange between Plants 
and Air. In 1730 there was born at Breda, in Holland, Jean 
Ingenhousz, who, after completing his university curriculum, 
began the practice of medicine first in Holland and afterwards, 
about 1764, in England. Four years later he became physician 
to the Emperor of Austria and resided in Vienna. His tastes 
seem to have been scientific rather than medical, for, during 


his life in the Austrian capital, he began to send papers to the 
Royal Society of London, of which he was elected a Fellow on 
his return to England in 1778. In 1779 Ingenhousz published 
his " Experiments on Vegetables/' carried out in his garden 
near London. 

In 1796 he published another work entitled " On the 
Nutrition of Plants and the Fruitfulness of the Earth/' in 
which he shows he had acquired a thorough grasp of the new 
chemistry founded by Lavoisier, a knowledge he confesses 
he did not possess when he wrote the " Experiments/' He 
knew now that carbon dioxide was a compound of carbon and 
oxygen, and this enabled him to realise the significance of 
the gaseous interchanges taking place between the green leaf 
and the air in sunlight. His new interpretation of the pheno- 
mena is that the leafy shoots give off oxygen in light and 
carbon dioxide in darkness, while non-green parts emit carbon 
dioxide both in the light and in the dark. From the carbon 
and the oxygen the plant constructs " acids, oils, mucilage, 
etc.," and these substances are then combined with the nitrogen 
of the air. This' latter idea is of course quite erroneous, but it 
took another fifty years to prove it so. That unfortunately 
was not the only blunder he made. Although he admitted 
that leaves absorb carbon dioxide from the air, he thought 
that a considerable amount also was obtained by the roots from 
the soil. Modern plant physiology teaches that all the carbon 
required by the ordinary green plant comes from the carbon 
dioxide present in very small quantities in the atmosphere, 
and the nitrogen from salts of nitric acid and ammonia in 
the soil. 

DE SAUSSURE. Nicholas Theodore de Saussure was the son 
of the celebrated naturalist, geologist and alpine explorer 
whom we have already mentioned as one of the pioneers in the 
science of geology. Theodore was born at Geneva in 1767, 
and was educated privately by his father. He studied medicine, 
mineralogy and natural history, and acquired a keen interest 
in chemistry from reading the works of Lavoisier. While still 
a lad he accompanied his father on his geological excursions, 
and learned from him habits of accurate observation of natural 


phenomena, an accompHshment rare enough in those days. 
When his father made his historic ascent of Mont Blanc in 
1787, he left his son at Chamonix, where Theodore employed 
himself in making meteorological observations, which after- 
wards proved of great value. In the following year^father and 
son spent more than a fortnight on the Col du Geant, at an 
altitude of over 10,000 feet, and would have remained 
longer had it not been that their guides, alarmed at the 
threatening condition of the weather, compelled the two 
scientists to descend by the simple, though drastic, method of 
destroying all the reserve stores of provisions. While the 
father studied the geology and meteorology of the Col and its 
surroundings, the son devoted himself to the determination of 
the exact altitude of the camp, the height of the neighbouring 
peaks, and the density of the air. Theodore next travelled in 
England and Scotland, making observations and laying the 
basis of his future work in vegetable physiology. In addition 
to researches on plant nutrition, he gave much time to problems 
in organic chemistry, such as the composition of ether and 
alcohol, and the nature and properties of various oils. He 
also took part in many public affairs, and was for several years 
a member of the Council of the Republic. Although appointed 
to the chair of mineralogy and geology in the Academy of 
Geneva in 1802, nothing could induce him to give a course of 
lectures. After a singularly uneventful life he died in Geneva 
in 1845. 

The Chemistry of Plant Nutrition. De Saussure's chief 
work was undoubtedly his " Chemical Researches on Vegeta- 
tion," a treatise published in 1804. In this book he proved 
that the seeds of plants will not germinate in the absence of 
air or oxygen, and that carbon dioxide retards germination 
owing to its poisonous effects on the tissues. He held that 
light had no effect on germinating seeds until green leaves 
appeared, and showed that a moderate addition of carbon 
dioxide to that normally present in the atmosphere favoured 
growth, provided that the intensity of light was increased 
correspondingly. He also showed that all green parts absorbed 
oxygen by night and restored it to the air by day, and by both 


qualitative and quantitative methods, physical and chemical, 
he determined which of the constituents of the plant were 
derived from the air and which from the soil. He claimed 
that plants obtained all their carbon from the carbon dioxide 
in the air, and none from the soil, as Ingenhousz thought. 
He was quite familiar with the importance of the green pigment, 
chlorophyll, but he fell into the error of believing that other 
colouring matters could act in its place. Thus he experimented 
with plants that had red or purple foliage, like orache or the 
copper beech, and observed that such plants gave off oxygen 
in sunlight, and he jumped to the conclusion that the green 
pigment was not essential to carbon assimilation, i.e. photo- 
synthesis, not noticing that chlorophyll was actually present 
though masked by coloured cell sap. 

De Saussure drew attention to the fact that in the daytime 
a plant reassimilates all the carbon dioxide it has formed in 
the process of respiration, and hence that respiration cannot be 
demonstrated while photosynthesis is going on. This explains 
what puzzled Mayow in 1674, when he found that green plants 
were not killed by being placed under bell-jars, although 
animals were. While Ingenhousz regarded water merely as a 
vehicle for the transport of salts from the soil to the leaves, 
de Saussure showed that water was decomposed by the leaves 
in sunlight along with the carbon dioxide. Growth without 
respiration, he said, was impossible; all the minerals taken up 
in solution by the roots had their parts to play in the plant 
economy, and, by means of a long series of what might be 
termed balance sheets, he arrived at a tolerably clear idea as to 
which minerals were essential, and how much of each was 
necessary for healthy nutrition. He made out one very im- 
portant point viz., that the plant derives all its nitrogen 
from the soil in the form of nitrates and compounds of ammonia, 
and none from the vast supplies in the air. He went astray, 
however, in regarding animal and plant waste as the source of 
these salts, and thus started what was called the "humus 
theory/' disproved towards the middle of the nineteenth 
century by the chemist Liebig. Lastly, de Saussure drew 
attention to the extreme dilution of the solution of salts taken 



in by the roots, and hence the necessity for the evaporation of 
the surplus water from the leaves transpiration, as it is called 

From this summary of de Saussure's work it will be seen 
that the publication of his " Chemical Researches on Vegeta- 
tion" constitutes a landmark in the history of Botany, and 
really forms the basis of almost all the work undertaken in plant 
nutrition up, at least, to the later years of the nineteenth 

Sensitivity in Plants. Another outstanding feature of the 
period was the publication of certain researches by a Hertford- 
shire horticulturist named Thomas Andrew Knight. Towards 

the close of the seventeenth cen- 
tury, Malpighi and others had 
observed the periodic movements 
of the leaves of some members of 
the pea family (Leguminosse), and 
Linnaeus and others had noted 
the responses given by the so- 
called " sensitive plant " as well 
as the curvings and twinings of 
tendrils and of stems like those of 
the hop and convolvulus, but in 
all cases these movements were 
ascribed to physical causes or to 
some mysterious " vital force/' 

Knight reopened the subject 
in a communication made to the 
Royal Society in 1806. He dis- 
cussed the persistent movement 
of the roots of seedlings towards 
the soil and of the shoots away 
from it, no matter how he disposed the seedlings in the first 
instance. By fixing seedlings on the rim of a rapidly rotating 
wheel (Fig. 89, A) he found that, if the wheel was horizontal, the 
roots bent downwards and the shoots upwards at angles diver- 
gent from the horizontal, which depended on the speed at which 
the wheel was rotating, while if the wheel was vertical 



(Fig. 89, B) all the shoots bent towards the nave of the wheel 
and the roots away from it. Knight's wheel was the forerunner 
of the instrument called a " Klinostat," now constantly used 
in every botanical laboratory. Knight tried to explain these 
movements by referring them to alterations in the position of 
the sap, but the explanation was by no means convincing. 

He also noticed that roots grew towards water even in 
opposition to gravity, and that dorsi-ventral leaves always 
tried to place their upper surfaces at right angles to the path 
of the sun's rays. " I will request your attention/' he says, 
" to the power of moving in the vine-leaf, on which I have 
made many experiments. It is well known that this organ 
always places itself so that the light falls upon its upper surface, 
and that if moved from that position it will immediately 
endeavour to regain it; but the extent of the efforts it will 
make, I have not anywhere seen noticed. I have frequently 
placed the leaf of a vine in such a position that the sun has 
shone strongly on its under surface; and I have afterwards put 
obstacles in its way on whichever side it attempted to escape. 
In this position the leaf has tried almost every method possible 
to turn its proper surface to the light." These words at once 
suggest a conscious effort on the part of the leaf to do something 
it was prevented from doing by some external agency. It is 
rather remarkable that Knight did not attribute to the leaf the 
power of perception or " feeling " so clearly indicated by the 
very words he uses to express the leaf's activities. " I am 
wholly unable to trace the existence of anything like sensation 
or intellect in plants," he writes;/' I cannot conceive how the 
contortions of its stalk, in every direction, can be accounted for 
without admitting not only that the leaf possesses an intrinsic 
power of moving, but that it also possesses some vehicle of 
irritation." Of course, " the vehicle of irritation " was there in 
the shape of protoplasm, but, as we have seen, that mysterious 
substance was not recognised as the basis of any such sensory 
phenomena until many years afterwards. 

Sensitivity in Animals. Sensitivity in plants suggests sensi- 
tivity in animals, and that introduces us to a man whose 
achievements have scarcely received the attention they deserve. 


Sir Charles Bell, as he ultimately became, was born in Edin- 
burgh in 1774, the son of a clergyman of the Episcopal Church 
of Scotland. After a brilliant career in Edinburgh, he went 
to London in 1804 and lectured on anatomy and surgery. In 
1811 he published his " Anatomy of the Brain/' and in that 
work, as also in subsequent papers on the nervous system, he 

proved the presence of sensory- 
nerve filaments carrying impres- 

G ^^^E^Ti^^S^. sions from the terminal sense 

organs to the brain, and motor 

^ nerve filaments passing from the 

FIG. 90. SPINAL NERVES. . r /T- x 

brain to the muscles, etc. (Fig. 90). 

These two sets of nerves are connected with the spinal 
cord by posterior and anterior branches, the former being 
sensory or afferent, and the latter motor or efferent. These 
and other researches on the nervous system brought him 
high honours, and his discoveries have been described as the 
greatest that had been made in animal physiology since Harvey 
demonstrated the circulation of the blood. He died suddenly 
in 1842. 

The Dogmas of Special Creation and Constancy of Species. 
When we were considering the works of Linnaeus, de Jussieu, 
de Candolle, Cuvier and others, we noted that they all firmly 
believed in the constancy of species, although vague doubts 
seemed to have been present in their minds as to whether some 
degree of variation ought not to be conceded within the limits 
of the species itself. Holding such views, the classifications 
they formulated could not, of course, be anything but linear 
arrangements of plants and animals, for a genealogical or 
phylogenetic tree was impossible. 

The first to call in question the special creation and specific 
constancy dogmas, at least in comparatively modern times, 
was Jean Baptiste de Monet, commonly known as the Chevalier 
de Lamarck. He was born at Bazentin in Picardy in 1744, 
and, unlike his elder brothers, who adopted the army as then- 
profession, he was destined for the Church. In consequence of 
this decision he became a pupil in the Jesuits 1 College at Amiens, 
where, however, he developed a pronounced distaste for theology 


and took the earliest opportunity of absconding and joining the 
army, then engaged in the Seven Years' War. As he arrived at 
camp, mounted on a derelict nag that he had acquired somehow, 
he was made a non-commissioned officer. His company suffered 
so severely in his first battle that Lamarck found himself, a lad 
of seventeen, in command of all that was left of it. But he 
showed the courage and determination that distinguished him 
throughout his whole career, for he resolutely refused to retire 
though facing great odds, until he had received definite orders 
from headquarters to do so. 

Invalided from the army owing to an accident, he went 
to Paris to study medicine, but from the beginning showed 
himself particularly attracted to botany. The fruit of five 
years' unremitting labour, carried out under poverty-stricken 
conditions and without any patronage or even encourage- 
ment, was his " Flora of France/* published in 1778. This 
work brought him to the notice of the Academy of Sciences, 
and after a short tenure of a subordinate post connected with 
that body, he was selected by Buffon as tutor to his sons. 
After two years of this life he returned to Paris, and was made 
keeper of the herbarium of the Royal Gardens, and finally 
professor in the renamed Jardin des Plantes. In 1794 Lamarck 
transferred his affections from botany to zoology, and im- 
mediately proceeded to reorganise the classification of the lower 
animals, which up till then had received the minimum of 
attention from zoologists, just as the cryptogams had been 
neglected by the botanists. He published his results in 1815 
and 1822 in his splendid work " The Natural History of Animals 
without Vertebrae/' Meanwhile he had become totally blind, 
and might have collapsed altogether had it not been for the 
devotion of his daughter, Comelie, who was his constant 
helper and sympathiser, while the outside world treated "him 
with indifference. Her prediction as to her father's future 
greatness came trueinlater years "La posterite vous honorera/' 
Lamarck died in 1829. 

The Doctrine of Use and Disuse. We need not do more 
than mention his views on the origin of life, for they were 
tinctured with an undoubted bias towards a belief in spon- 


taneous generation, but will turn to his theory of evolution, 
which, although vigorously combated and even ridiculed, was 
the first serious attempt to overthrow the dogmas of special 
creation and the constancy of species. His general theory is 
given in the introduction to his " Natural History," and very 
briefly is that: variation is explicable on the ground of use 
and disuse; the constant use of an organ tends to its develop- 
ment and its disuse to its degeneration and ultimate abortion; 
every alteration in an organism so arising is passed on to its 
offspring, either to be maintained by it or to be allowed to fall 
into abeyance; acquired characters are inherited. 

Whatever else may be said for or against Lamarck's theory, 
we must give him the credit of recognising the two fundamental 
factors that every evolutionist must take into account, how- 
ever he may explain them viz., the phenomena of variation 
and the phenomena of heredity. 

To Lamarck also we owe the introduction of the word 
"biology/' to signify the science which deals with living 
organisms, whether they be plants or animals. 

CHARLES DARWIN. The year 1859 was destined to be a 
fateful one not only for biology but for every department of 
learning, for in that year was published a book that has had a 
deeper and more far-reaching influence on the trend of human 
thought than any other that has ever come from the Press. 
That book was the " Origin of Species/' by Charles Darwin. 
Its only competitor is the " Principia " of Sir Isaac Newton, 
and he and Darwin may rank together as the two greatest 
scientific men that the world has ever seen. 

Charles Robert Darwin was born in 1809, at Shrewsbury, 
where his father was a well-known medical man. It is rather 
amusing to read how Darwin's early teachers looked on him as 
not far removed from a dunce ! These were the days when 
higher education was considered as synonymous with an intimate 
knowledge of the classics, and it is on record that his headmaster 
on one occasion publicly reprimanded him for " wasting his 
time on such a contemptible subject as chemistry." " The 
school as a means of education to me was simply a blank/' he 
writes himself* He does not appear to have been much more 


of a success at college. At Edinburgh, where he went to study 
medicine, he found the professors " intolerably dull/ 1 and 
speaks of some of the lectures as " fearful to remember/' As 
Huxley puts it: " The climax seems to have been attained by 
the professors of geology and zoology, whose prelections were 
so incredibly dull that they produced in their hearer the some- 
what rash determination never to read a book on geology or 
in any way to study the science so long as he lived/' This, be 
it remembered, was the decision of the future author of the 
" Structure and Distribution of Coral Reefs," and of the " Geo- 
logical Observations on Volcanic Islands " ! 

Finding himself not likely to become a successful physician, 
Darwin exchanged Edinburgh for Cambridge, where he entered 
Christ's College with the view of reading for the Church; but 
he had no better word to say for Cambridge than he had for 
Modern Athens. " During the three years which I spent at 
Cambridge/' he writes, " my time was wasted, as far as 
academic studies were concerned, as completely as at Edinburgh 
and as at school/' If he passed the door of the geology 
lecture-room with a shudder, he found his way into the botanical 
one, and there he made the acquaintance of Henslow, " a man 
of rare character and singularly extensive acquirement in all 
branches of natural history/' This acquaintance ripened into 
a friendship which lasted till Henslow's death in 1861. Henslow 
overcame Darwin's prejudice against geology, and succeeded 
in introducing him to Sedgwick, at that time professor of 
geology at Cambridge, and Darwin had to forswear his vow 
never to study the science again, for he not only accompanied the 
professor on his geological excursions, but set himself to master 
LyeJTs " Principles of Geology," a work to which, in after years, 
he professed himself as fundamentally indebted* 

Henslow was, however, responsible for more than merely 
turning Darwin's thoughts to botany and zoology; he showed 
himself possessed of a far-seeing vision that was the means of 
dedicating to science the man who was destined to become 
the greatest of her high priests. For it was due to Henslow 
that Darwin, when scarcely out of his teens, was appointed 
naturalist on H.M.S. Beagle, just about to start on a five years' 


surveying voyage round the world. Thus another educational 
experiment was to be tried, and Nature herself was to take him 
in hand. This last attempt at instructing the future naturalist 
was as successful as the others had been futile, and was, as 
Darwin himself says, the starting-point of his second life. 

The Theory of Evolution. In a letter written in 1877, 
Darwin says, " When I was on board the Beagle I believed in 
the permanence of species, but, as far as I can remember, 
vague doubts occasionally flitted across my mind. t)n my 
return home in the autumn of 1837, I immediately began to 
prepare my journal for publication, and then saw how many 
facts indicated the common descent of species, so that in July, 
1837, I opened a notebook to record any facts which might 
bear on the question. But I did not become convinced that 
species were mutable until, I think, two or three years had 
elapsed/ 1 

Soon after his return from his travels, Darwin came across 
a book caEed " An Essay on the Principles of Population," 
written by the Rev. Thomas Malthus, a Surrey vicar, who had 
died while Darwin was abroad. In this volume, Malthus argued 
that organisms, if left perfect freedom to breed and unlimited 
room in which to multiply, would fill any conceivable area in 
a very short time. The only limits to their increase were space 
and food. So far as man was concerned, propagation was 
controlled by reason, although in his case also the same 
limitations were existent and active, and these he proceeded 
to enumerate and illustrate from the histories of different 

Darwin wrote out his theory in 1844, but, instead of pub- 
lishing it at once, he spent the next fifteen years gathering data 
bearing on the various aspects of the subject, conducting experi- 
ments and, so to speak, polishing the rough marble into the 
perfect statue. During this period also he was busily engaged 
in working out other problems suggested by, or connected with, 
his main thesis, the gist of which latter was known only to a 
few very intimate friends. At length, in 1856, at LyelTs 
request, Darwin began to expand the preliminary sketch of his 
theory, which he had written in 1844, so as to bring it into book 


form, but with no immediate intention of publishing. In May, 
1857, he wrote: " I find the subject so very large, that though 
I have written many chapters, I do not suppose I shall go to 
press for two years." 

. But earlier publication was forced upon him by the ap- 
pearance of a new worker on the same problem. This was 
Alfred Russel Wallace, a young architect, who had some years 
previously exchanged his practice for a life of travel and ex- 
ploration in Brazil and Malaya, and with whom Darwin had 
had some correspondence on natural history subjects. Wallace 
tranmsitted to Darwin an essay entitled " The Tendency of 
Varieties to depart indefinitely from the Original Type/' the 
outcome of his observations and meditations amid the tropical 
forests of the Eastern Archipelago, and which, as Darwin says, 
was in effect an admirable abstract of his own unpublished 
work. After consulting his two most intimate friends, Lyell 
and Hooker, he decided to publish Wallace's essay and an 
abstract of his own book simultaneously, and the two papers 
were read to the Linnean Society on July i, 1858 a truly 
historic date, and one to be remembered by every student of 
science. A little over a year later appeared the famous volume 
" On the Origin of Species by Means of Natural Selection, or 
the Preservation of Favoured Races in the Struggle for 

The Origin of Species. Perhaps the easiest way to obtain 
a grasp of the theory expounded in this remarkable work 
" one of the hardest books to master," as his disciple Huxley 
called it is to follow the outline given by Wallace in an article 
on " Creation by Law," which appeared in 1868 in the Quarterly 
Journal of Science. Wallace's demonstration is in the form 
of a table consisting of two parallel columns, the first containing 
proved facts, the second, legitimate deductions from these 
facts, which deductions are afterwards transferred to the first 
column and used as proved facts in turn. The table is a very 
brief one, but each sentence really represents a volume. It 
may be well to give the table just as Wallace published it, and 
add a few sentences by way of explanation. 


A Demonstration of the Origin of Species by Natural 


3. Straggle for existence. ( 5- Survival of the fittest, or 

4. Heredity with variation, or J natural selection i.e., on the whole, 
general likeness with individual differ- 1 those die who are least fitted to 
ences of parents and offspring. (, maintain their existence. 

7. Changes of organic forms to 
keep them in harmony with the 

5. Survival of the fittest. 

6. Change of external conditions, 
universal and unceasing. 

changed conditions; and as the 
changes of conditions are permanent 
changes, in the sense of not reverting 
back to identical previous conditions, 
the changes of organic forms must be 
in this sense permanent and thus 
. originate species. 

Beginning with " proved facts," we have first the undoubted 
fact that organisms, if left to themselves and given abundant 
food and space in which to multiply, increase at an enormously 
rapid rate. Huxley on one occasion made a calculation that 
if a plant required I square foot of ground on which to live, and 
if it were given a " fair field and no favour," and if it produced 
only fifty seeds a year, which could be effectively scattered over 
the surrounding ground, then in ten years, assuming all the 
plants survived, there would be 1,953,125,000,000,000 plants in 
existence of this particular kind, and since the whole land area 
of the globe amounts to about 1,421,798,400,000,000 square 
feet, there would not be nearly sufficient space to hold them ! 

The second fact is that the total number of individuals of any 
species remains, on the whole, fairly constant. If there be, 
say, 1,000 dandelion plants on -a neglected lawn in any one 
year, and if each plant produces 100 seeds, we do not expect 
to find 100,000 dandelions in the year following; there may be 
1,100 or only 900, but the number remains approximately the 
same. The obvious conclusion is that there is a constant 
struggle for existence going on, quite unconscious, of course, 
in which the deaths on an average equal the births. 

Accepting this struggle for existence as a proved fact and 
transferring it to the first column, we note the admitted facts 


that offspring, although resembling their parents in all 
essential particulars, show individual differences of their own; 
in short, we are forced to accept the facts of heredity and 
variation. The conclusion to be drawn is that, on the whole, 
those of the offspring that show any variation which gives 
them an advantage, however slight, over their neighbours are 
more likely to survive. This we term survival of the fittest, or 
natural selection. 

Finally, accepting survival of the fittest as a proved fact, 
and coupling with it the known fact of the universal and 
unceasing changes in external conditions, we draw the con- 
clusion that there must be corresponding changes in organic 
forms to keep them in harmony with these changing conditions. 
But these changes in external conditions never revert to identical 
previous conditions, and so the changes in organic forms must 
also remain permanent until the environment alters, 

" It is doubtful/' writes Huxley, " if any single book, except 
the ' Principia/ ever worked so great and so rapid an evolution 
in science, or made so deep an impression on the general mind. 
It aroused a tempest of opposition and met with equally ve- 
hement support, and it must be added that no book has been so 
widely and persistently misunderstood byboth friends and foes/ 1 

During his voyage in the Beagle Darwin suffered much from 
the effects of a serious illness contracted at Valparaiso, and this 
left its mark upon him to such an extent that, as he tells us in 
his autobiography, he never really recovered his initial health 
and strength. " My chief enjoyment and sole employment 
throughout life has been scientific work, and the excitement 
from such work makes me, for the time, forget, or drives quite 
away, my daily discomfort. I have, therefore, nothing to 
record during the rest of my life except the publication of my 
several books/' After 1842 Darwin removed from London to 
a country residence at Down in Kent, where he spent the re- 
mainder of his years. That historic residence has lately been 
purchased as a national property to be held in remembrance 
for ever of its illustrious occupant. 

Darwin's other Biological Works. In addition to the 
" Origin/' Darwin contributed many books and other publica- 


tions on almost every branch of natural history, any one of 
which would have earned him distinction had they not been 
overshadowed by the great classic that has made his name 
immortal. He wrote : "On the Various Contrivances by which 
Orchids are Fertilised by Insects/' " On the Effects of Cross 
and Self-Fertilisation in the Vegetable Kingdom/' " On Different 
Forms of Flowers on Plants of the Same Species/' "On In- 
sectivorous Plants/ 1 "On Climbing Plants/' and "On the 
Power of Movement in Plants/' In 1863 he published " The 
Variation of Animals and Plants under Domestication/' really 
an expansion of the first chapter of the " Origin/' and later, 
a volume on " The Descent of Man/' a book which created 
almost as great a sensation as the " Origin " itself. 

In the beginning of 1882 Darwin's health, always feeble, 
began to give way very rapidly, and he breathed his last on 
April 19 of that year, at the age of seventy-three. 

It had been the wish of his family that he should be buried 
at Down, but they bowed to the widespread feeling that a man 
so illustrious should find his resting-place in the national 
Valhalla; so on April 26 his funeral took place in Westminster 
Abbey, attended by representatives of all the great nations, 
as well as by very many distinguished personages and personal 
friends. Appropriately enough he lies only a few feet away 
from his only compeer, Sir Isaac Newton, and his tomb bears 
the inscription he would have regarded as most fitting in its 




" The lapse of time with the truer proportions that distant 
vision gives will show the figure of Charles Darwin towering 
alone above all others in the history of philosophy " (Graham 


IN the preceding pages we have followed the story of scientific 
discovery from the earliest times down, approximately, to the 
middle of the nineteenth century. We have seen the bud 
opening in the spring when the giant form of Sir Isaac Newton 
" turned darkness into light," and we have traced the evolution 
of the young shoot into the leafy branch, but we recognised also 
that in the axil of each leaf there lay another bud destined to 
unfold in its turn. 

Some of the most striking of the developments we have 
witnessed during our own lifetimes have been in the region of 
inventions i.e., the applications of scientific knowledge to 
matters of everyday life. Even to enumerate these would 
occupy far too much space, so that we must confine ourselves 
to sketching very briefly some of the chief discoveries in pure 
science, leaving the detailed story of their applications to those 
specially competent to deal with such matters. 

The Interrelationships of the Sciences. At the very outset 
we have to recognise that it is impossible to separate the 
advances in one science from those in another. Astronomy, 
for instance, has taken advantage of discoveries in physics to 
measure the distances of the stars and to analyse their com- 
position; physics and chemistry have become so inextricably 
blended that we now have professorships of physical chemistry 
in our universities; geologists have called in radioactivity to 
aid them in determining the age of the earth; chemistry and 
biology have joined hands in bio-chemistry; in short, the 
boundaries of the different sciences are fast breaking down, 
and the most interesting of the problems yet unsolved lie in 
these borderlands where two or more sciences meet. 

The Multiplicity of Workers. Where a century ago com- 
paratively few workers were tilling the fields of science, there are 
now thousands, the great majority, perhaps, adding very 
minute fragments to the rapidly growing mass of data; but a 


few greater minds are arranging these data into co-ordinate 
wholes and deducing general principles from their study. Any- 
one who has undertaken research in a scientific subject is only 
too conscious of the difficulty of collecting and digesting the 
publications bearing on the problem on which he is engaged, 
and that fact is brought home to him in a striking manner by 
the contemplation of the volumes published in any one year 
which merely enumerate the titles of works dealing with 
any one of the sciences. 

The Unity of Science. At a recent meeting of the British 
Association for the advancement of Science (1926), the dis- 
tinguished head of the astronomical department of Cambridge 
University, Professor A. S. Eddington, made a very interesting 
and striking comparison between three things which at first sight 
do not appear to have much in common viz., a star, an atom 
and a man. A star like our sun, he told his audience, belonged 
to a system embracing some 300,000 million stars, most 
of them with diameters measurable in hundreds of thousands 
or even millions of miles. These stars, he said, were scattered 
through space at inconceivable distances apart, and, further, 
that our whole stellar system was one of many, for nebulas 
may be " island universes " far outside the galaxy to which our 
system belongs. A drop of water, he told us, contained 
"several thousand million million million atoms/' each of 
which was probably one hundred-millionth of an inch in 
diameter, and yet within the atom are electrons whirling 
round a central nucleus, like planets round a sun, and just as 
far from it comparatively as the planets are from our own 
sun. On the one hand we had the infinitely great, and on 
the other the infinitely small. About midway between these 
two extremes came man. His body consisted of atoms so 
numerous that the number could be expressed only by ten 
followed by twenty-seven noughts, while ten followed by twenty- 
eight noughts might represent the number of human bodies re- 
quired to form a star. So " from his central position man can 
survey the grandest works of Nature with the astronomer, or 
the minutest works with the physicist. The road to a know- 
ledge of the stars leads through the atom; and important know- 


ledge of the atom has been reached through the stars" (Stars 
and Atoms). 

What discoveries the years to come may hold in store for us 
we know not, but we may feel assured that, just as a stone 
dropped from a height falls more and more rapidly from its 
point of departure until it reaches the earth, so science may 
advance with ever-increasing strides from year to year. 


As the subjects dealt with are to be rendered intelligible to 
those who have had no previous mathematical training, it will 
be necessary to simplify the treatment of each as much as 
possible, so that the reader may not feel discouraged by being 
suddenly confronted with formulae, simple enough, perhaps, to 
the expert, but conveying little or no meaning to the average 
individual. But in reading what follows it will be found con- 
venient and helpful to consult from time to time the table of 
useful data given at the end of this book (p. 466). 

It is no exaggeration to say that if a student, reasonably 
conversant with the problems of physics and chemistry as 
they were taught fifty or sixty years ago, had fallen asleep 
then, and, like Rip van Winkle, had awakened in the year 1930, 
he would have found himself in another world, and it would 
have taken him much time, labour and thought to adjust 
himself to the new outlook. It is only by wholesale omission 
of many important developments that we can hope to crowd 
into a few pages more than a fragment of the new knowledge 
of Nature and her works, that has been wrested from her by the 
efforts of the brilliant scientists of the present generation. All 
that we dare aim at, is to outline the trend of modern enquiries 
into what the Roman poet, Lucretius, called the " nature of 
things/' To attempt to do more would be to defeat the very 
aim of this book, and to lose sight of the wood in the contempla- 
tion of the individual trees. 


Heat and the Kinetic Theory. 

We have seen (pp. 117-122) how slow was the emancipation of 
the scientific mind from the old doctrine that heat was a sub- 
stance (" caloric "), and how gradually it became recognised that 
heat was merely a special mode of motion, and was eventually 
(pp. 130, 131) identified as one of the forms of energy. Kinetic 
energy is the name of this energy due to motion, being, for all 
ordinary velocities, in fact proportional to the mass of the moving 
object and to the square of its velocity. This simple law can 
be put E= Jwv 2 , where E is the kinetic energy, m the mass, and 
v the velocity, and this law is true of all material objects moving 
at any speed, except such as approaches the velocity of light 
(see p. 295). 

For instance, a motor-car weighing 2 tons travelling at a 
speed of 20 miles an hour has twice the kinetic energy of one of 
i ton at this speed. It will do twice the damage in a collision and 
require twice the braking-power to stop it within a given distance. 
But if one of two cars of the same weight is travelling at twice 
the speed of the other, say 40 miles an hour as against 20 for the 
other, the faster car will possess 2 2 , that is 4 times the kinetic 
energy, and if this energy unfortunately took the form of 
smashing things up in a collision, there would be 4 times the 

Exactly the same thing, on a vastly smaller scale, applies 
to the minute particles or units of matter which we call molecules, 
and so long as they are sufficiently free as to be able to move 
finite distances, as in a gas, it can be shown that these infini- 
tesimally small particles follow the above law. The motion of 
these particles is equivalent to heat the higher the temperature 
the greater the velocity; and moreover, heat is a mode of motion 
(more restricted in the case of solids and liquids) for every form 
of matter. It is a very simple conception, and yet among the 
great triumphs of the nineteenth century must be numbered 
this recognition, that it is the vibratory and rotatory motion of 
ordinary molecules that endows matter with the heat which 
*we define by temperature, and gases with their expansive tenden- 


cies. This is the kinetic theory, the mathematical elucidation 
of which is principally due to Clausius and Clerk-Maxwell (p. 476) . 
Long before the actual sizes or shapes of molecules were known, 
the calculations were made on the simple assumption that they 
consisted or behaved as small elastic spheres, which, by their 
motions and collisions, increasing as the temperature increased, 
produced not only the effect of temperature but also the effect 
of pressure, by the continual bombardment of the walls of any 
vessel containing them. These conclusions have been abundantly 
verified by the later researches of the twentieth century, and 
we know now a great deal about the sizes and shapes of molecules, 
their prodigious numbers under ordinary conditions even in the 
smallest bubble of gas, their " free path " which is the average 
distance of run before colliding with another molecule, the 
enormous number of collisions per second, and so on. 

It would be beyond the scope of this book to give the mathe- 
matical or experimental proofs derived from the mass of data 
which has been accumulated; and in any case the magnitudes 
involved are so enormously great or small, as the case may be, 
that they convey meaning only to the mathematician or physicist* 
For example, the number of molecules at o C. and normal baro- 
metric pressure (760 mm.) in i ex. (p. 466) is the same for all gases * 
(Avogadro'slaw) andamountsto 27 Xio 1 *, which means 27 mul- 
tiplied by i followed by 19 noughts. The mean velocity is dif- 
ferent for different molecules (greater for light ones like hydrogen, 
being, as the formula on p. 224 indicates, inversely proportional 
to the square root of weight, m, of the molecule divided by 2) ; it 
is about a mile a second for hydrogen at ordinary temperature - 
and about 500 yards a second for the component molecules 
of air. With the crowded numbers and these high speeds, it 
will be readily imagined that the molecules cannot get very far 
without a collision, and since for different molecules, at a given 
temperature and pressure, the numbers are the same while the 
speeds are not, the free path will depend on the gas. For 
hydrogen molecules it is less than -nnfanr centimetre; but if 
the pressure is reduced (and so the number of molecules) the free 
path will be correspondingly greater. At the lowest attainable 
vacua of the laboratory there are still millions of molecules per 


cubic centimetre, and the free path is about 100 yards. The 
tenuity of nebulae (see p. 271) is so slight that molecules have 
an enormous range, which may amount to millions of miles 
before they collide with another this in the outer rarefied 

A sudden reduction of pressure on any gas causes a fall of 
temperature because of this increase of free path there is less,. 
agitation in a given volume; and conversely compression, which 
reduces the free path and causes greater agitation in a given 
volume, heats the gas. This temperature fall or rise due to 
change of pressure is known as the adiabatic change, and it has 
great importance in connection with the physics of the atmo- 
sphere. This is dealt with on p. 366, and it is sufficient here to 
point out that the change is strictly reversible and of the utmost 
value for taking away heat from a gas, when it is desired to 
reduce the temperature greatly, say for purposes of liquefaction 
by strong cooling. Thus if a gas is compressed in a perfectly 
insulated vessel there is a rise of temperature (which can be 
calculated with great accuracy from the adiabatic laws), and if 
now the pressure is reduced to what it was initially the tempera- 
ture falls back to what it was initially; but if the compression- 
heat is withdrawn (by cooling) before decompressing the 
temperature will be lower than what it was initially, after the 
pressure is removed. And by repeating the process all gases 
can be so cooled as to liquefy them, 

Adiabatic Laws. The adiabatic laws are really derived 
from the law of Boyle (p. 50), which states that the pressure 
is inversely proportional to the volume, if the temperature is 
adjusted so as to remain constant, and from the law of Charles 
and Gay Lussac, which states that the volume is proportional to 
the temperature in absolute (not Centigrade) degrees, if the 
pressure is adjusted so as to remain constant. For example, 
if a litre of any gas at o C. and 760 mm. pressure (30 inches of 
mercury) is heated to 273 C., the pressure being kept constant 
by allowing the gas to expand, the volume will then be 2 litres, 
because the absolute temperature has been doubled (since the 
initial temperature o C.=273 absolute). 

We can now better understand the absolute scale of tempera- 


ture, since it is from the zero absolute (-273 C.) that tempera- 
ture-functions in all laws of physics start it is indeed the zero at 
which aE heat vanishes i.e., all molecular motion. We cannot 
reach absolute zero in the laboratory, though points have been 
touched only a little above by adiabatic cooling, involving the 
successive liquefaction of air, hydrogen, and last of all helium, 
which has the lowest boiling-point of any known substance. 

Theoretically by Charles and Gay Lussac's law a gas on 
cooling to this zero temperature should vanish to zero-volume, 
which is of course absurd. In practice this does not happen, 
for the reason that the law ceases to hold before zero is reached 
by a change in state of the gas, to the liquid (or solid) condition. 
In its simplest form the combined Boyle-Charles-Gay Lussac 
law may be stated in the form, that the product of the pressure 
multiplied by the volume is equal to the absolute temperature 
multiplied by a constant (R), and it is commonly written: 

But this simple equation takes no note of factors which are 
ordinarily present namely, the actual size of the molecules 
themselves (small in comparison to the bulk of the gas it is true, 
but yet not infinitely small) and a certain attraction (a sort of 
adhesive attraction) which these molecules have for one another, 
which is very noticeable when their concentration becomes great, 
near the point when the gas is going to liquefy. When these 
corrections, which were first elucidated by van der Waals, are 
incorporated in the above law a complete^e^ialion* 8 ^!! be 
deduced, and from this foundation the adiabatic laws can be 
built up. The mathematical expression of these laws, together 
with the second law of thermodynamics (p. 131) has led to an 
elaborate treatment of the pressure-temperature changes in 
the atmosphere, involving a special conception of entropy, which 
it is beyond the scope of this book to deal with ; and in the hands 
of the astro-physicist they have enabled the constitution of the 
stars to be elucidated with great nicety. 

Liquids. Although the greatest triumphs of the kinetic 
theory are related to researches on gases, it must be remembered 
that this theory has greatly facilitated the elucidation of the 


nature of liquids and solids. The distinction between the three 
fundamental states of matter viz., gas, liquid, and solid is 
essentially a distinction in the concentration of the molecules 
and their degrees of freedom, due to the physical conditions 
(temperature and pressure) imposed, 

A state of matter, in scientific language, is known as a phase, 
and certain well-defined principles have been elucidated by the 
American physicist Willard Gibbs, governing the conditions 
of coexistence of separate phases and their mutual transition 
into one another. All that is necessary here is to try and explain 
what is the difference between a solid, liquid and gas. In terms 
of the kinetic theory it is easy to see that there is something 
approaching perfect freedom in the case of gas molecules. A 
" perfect " gas is not known, though hydrogen approaches this 
condition, it being understood by the word perfect that the 
above laws of Boyle, Charles and Gay Lussac hold for all con- 
ditions of temperature and pressure; that in fact the individual 
molecules should behave as perfectly elastic spheres of zero- 
volume and having no physical attraction for each other. In 
so far as these two conditions are never quite realised, a gas 
shows smaller or greater deviation from the gas laws and behaves 
as a jostling crowd of molecules, showing some though small 
interest in each other, like a swarm of bees. Their movements, 
however, are free, irregular, and vigorous; the molecules are in- 
dependent and perform their own zig-zag course as they collide 
with others, but they are not otherwise restricted. 

A liquid, on the other hand, is in very different case. Here 
the molecules find themselves in virtual contact owing to the con- 
ditions prevailing; they are not free, simply because they cannot 
escape each other's influence, though they have sufficient 
freedom to work their way slowly between other molecules into 
new positions, this motion being more active the higher the 
temperature. It is like a closely packed crowd of human beings, 
where each individual is trying to worm his way to some other 
position, whereas a gas is more like a thinly dispersed gathering 
of people, running to and fro, on some open heath, with plenty 
of room up to a point, but not sufficient to prevent constant 
collisions. Perhaps a better analogy for the liquid condition is 


a canister full of fine polished leaden shot, kept in a continual 
state of agitation by some imaginary stirring agency within. Such 
particles continuaiy flow over each other, they can be poured 
out of the vessel en masse, but they will not cohere, 

Solids. A solid is still more restricted so far as molecular 
freedom is concerned. The units of the crowd here are literally 
tied together, just as if a mob of human beings joined hands in 
such a way as to restrict all independent movement. The people 
can " jiggle " about, bob up and down and even twist round to 
some extent, but they cannot get away from each other till they 
loose hands. This is only a rough analogy to the solid or crystalline 
state, but it has been disclosed as applicable to molecules by the 
penetrating researches of Sir William Bragg and his followers, 
using the method of X-ray analysis. This X-ray photographic 
method actually reveals the positions of the atoms and molecules 
in solids, their distance apart (of the order of a few Angstrom, 
units see p. 259), the angle of the imaginary lines joining them, 
and so on. It is a wonderful story, and the surprising feature 
of the crystal lattice of compounds, so revealed, is that the 
molecules, as such, seem to be less apparent as units than are 
the atoms comprising the molecules. Did we not know from 
other sources that the molecules are the fundamental units, we 
should almost imagine from X-ray analytical methods that the 
units were the atoms. It is, of course, the effect of molecule- 
packing in definite-shaped contiguous cells which repeat 
themselves, but in which the atoms show up in the photographs, 
that gives this rather erroneous impression. And, of course, 
it must not be imagined that these atoms are seen they are, of 
course, much too small. What is seen is their effect on X-rays 
in producing figures by which their positions and distances can 
be identified and measured. 

A crystalline solid, then, is a molecular structure, in which 
the atoms are spaced in a practically fixed position, so as to 
form some geometrically shaped group or cell, specific for each 
substance, a cell which is endlessly repeated throughout the 
mass of the crystal. The number of such cells in a gram (p. 466) 
of solid is of course vast, but known in many cases. The number 
may be gathered from the case of water, one gram of which 


contains 3-3 Xio 22 molecules (3-3 X i followed by 22 noughts), 
and when water crystallises to form ice these molecules group 
themselves into cells of a few molecules, each packed into a 
rather loose type of hexagonal lattice. In the latter, each oxygen 
atom is at the centre of gravity of four neighbouring equidistant 
oxygen atoms, from which it is separated by a hydrogen atom, 
so that two of the latter are found in conjunction with each 
oxygen atom. 

The same kind of thing is found in all crystalline solids, 
and if it is asked what are the forces binding the molecules 
together in these geometrically shaped cells, the answer is to be 
found by appealing to the same kind of attractive force which 
was referred to (p. 227) in the case of molecules of gases, and 
which tends to make them " imperfect." It is a force quite 
different from, though doubtless related to, that which binds 
the atoms themselves together in forming the molecules. This 
force binding atoms, known as chemical affinity, is electrical, 
and, though not yet completely understood, it is in some way 
connected with the movements of electrons which surround the 
nuclei of atoms, and of which we shall have a good deal to say 
later on (p. 246). The force binding molecules together is 
probably also electronic, being in the nature of so-called residual 
affinity, and it is probably of the same kind as that which we 
call ordinary adhesion. 

These forces, tending to bind molecules together, are in op- 
position to heat, which by endowing the molecules with greater 
kinetic energy tends to loosen them. In liquids both opposing 
forces are in equilibrium, and so the molecules of liquids have a 
considerable degree of freedom. But the moment the kinetic 
energy falls to a certain value 4.e. t at a perfectly definite 
temperature (the freezing -point) for each substance it ceases to 
effectively oppose the forces of residual affinity of the molecules, 
and the liquid solidifies. The molecules of solids still have, 
however, some degree of freedom within their apparently rigid 
crystalline lattice, so long as the temperature is above absolute 
zero, and it is this restricted vibratory movement (in the three 
dimensions of space) which confers upon them the property of 
being warm. 


Viscosity. There are certain types of liquid which do not 
crystallise readily, if at all, when cooled, but instead become 
thicker and more treacly, and eventually may appear to be 
solid, though not truly so in the crystalline sense. We all think 
of glass as a solid, or even toffee, but they are rather to be re- 
garded as liquids of very high viscosity, since they possess none 
of the properties of crystalline solids. Such substances are said 
to be amorphous. The case is better illustrated by considering 
liquid viscosity more closely, a property due to those forces 
referred to already, tending to bind the molecules together and 
dependent largely on the chemical nature of the liquid substance, 
but greatly affected by temperature. For example, the chemical 
compound known as common ether (used in anaesthesia and also 
as a solvent) has a very low viscosity at ordinary temperatures 
much lower than that of water, for instance. It is much more 
mobile than water and will run through a narrow orifice or tube 
much more rapidly. Also, water when hot may be six times 
more mobile than when cold, as shown by the more rapid rate at 
which it will pass through a narrow orifice its viscosity is reduced 
by rise of temperature, as is the case with all liquids (provided 
something else does not happen to interfere with this change). 
Common pitch is quite a mobile liquid at about 180 C., but as the 
temperature falls the liquid becomes thicker and thicker, till 
when cold its viscosity is so great that pitch behaves very like 
a solid in its resistance to deformation. Glass is similar, and 
even water and many other liquids will behave similarly and 
assume a glassy condition (amorphous) if cooled well below 
their freezing-points, provided that certain conditions are 
observed, difficult to attain, to prevent crystallisation, which 
is the normal behaviour of such liquids. It is really a matter 
of time, because, if the temperature can be got many degrees 
below the freezing-point quickly enough, the molecules at this 
low temperature are now so sluggish in their movement that it 
takes a very long time for them to arrange themselves in the 
orderly geometrical pattern of crystals; indeed, the colder the 
glassy form is the more stable it will be. 

The preceding description of the three fundamental states 
of matter is of course a very simplified one, and omits all chemical 


considerations including influence of admixture. In practice 
the whole problem is, of course, highly complex, but, nevertheless, 
the simple picture portrayed is tolerably faithful for purposes 
of illustration. It means that, apart from accidental complica- 
tions, any piece of matter can be imagined (and frequently 
induced) to exist in all three states or phases solid, liquid, and 
gas or vapour. [The term vapour is a loose one, generally, 
however, connoting the gaseous condition near the point of 
condensation to liquid.] That is to say, the state or phase of a 
substance is determined by the conditions (temperature and 
pressure) imposed. A substance may appear as a solid, as a 
liquid, or as a gas, subject, however, to the restriction that it is 
sufficiently stable, chemically, overthe temperature rangeinvolved 
i.e., to remain undecomposed. Thus cellulose (in wood) is a 
solid, and it cannot be liquefied or converted into gas, in the same 
kind of way that ice can be converted into water and steam, for 
the reason that its molecules are relatively unstable chemically, 
and if you heat them too much in the attempt, say, to gasify 
them, you simply break up these molecules into a heterogeneous 
crowd of other molecules by chemical decomposition. Apart 
then from this, as it were, accidental factor, matter normally 
is capable of existing in three states, according to the temperature 
and pressure. Low temperature or high pressure tends to induce 
the solid or liquid condition; the converse tends to induce the 
gaseous condition. But, as might be expected, the chief factor 
determining the state of matter, under conditions obtaining on 
earth, is the nature of the matter, that is to say the kind of 
molecules, their size and mass, and the chemical or physical 
attraction they have for each other. And so it comes about 
that there is an endless diversity in the kind of molecules with 
which we are familar as ordinary matter, say, on earth, and 
a corresponding endless variety of physical (and chemical) 

Broadly speaking the simpler and less massive molecules, 
like those of, say, oxygen or water, will have a tendency towards 
the gaseous state, and require more or less high pressures and low 
temperatures if they are to present themselves as liquids or solids. 
On the other hand, the more complex molecules, like those of 


sand (silica), iron, carbon, etc., require high, temperatures in order 
to appear as gases. At the temperature (about 6,000 C.) of the 
sun's surface everything is gasified, and chemical compounds 
would be decomposed or dissociated into their elements, and so 
the molecules there practically all consist of elementary atoms. 
To illustrate better the changes of state with varying tempera- 
ture and pressure let us take a very simple example, water. 
Imagine a little water contained in an hermetically closed glass 
tube containing nothing else (i.e., all air originally in the tube 
having been extracted). There is an empty space above the 
water in the tube, which is not strictly empty but filled with 
mobile molecules of water as invisible gas or vapour (steam). 
And if we measure the pressure of this gas in the tube, as we 
could do by fixing a manometer (p. 69), we should find that 
the pressure increases as the temperature is increased, and that 
there is indeed for every temperature an exactly corresponding 
pressure. This is known as the vapour pressure of the liquid 
(whatever it is, for the principle applies to all cases) . Moreover, 
as we raise the temperature we find that some of the liquid disap- 
pears, and a corresponding increase occurs in the concentration 
of the steam molecules in the space above. [Concentration is 
a term of great significance in science: it denotes the weight or 
total mass in unit volume.] When we reach a temperature of 
100 C. in our closed tube the pressure is found to be exactly 
760 mm., which is that of the atmosphere (average) at sea level 
i.e., one atmosphere or normal barometric pressure. If we 
go on heating above 100 and the tube is strong enough to 
withstand the increasing pressure, nothing apparently happens 
to the liquid water (it does not boil) except that it diminishes in 
quantity whilst the steam concentration increases. In fact, the 
pressure increase now becomes very much more rapid than that 
of the temperature increase. It was so all along and the two 
are not proportional, but at high temperatures the pressure effect 
becomes steeper and steeper. And a very strange thing happens 
when 365 C. is reached. At this point the surface of the liquid 
water becomes hazy and not sharply defined as it was before; 
and if any attempt is made to raise the temperature beyond 
this, the liquid simply disappears and the whole tube is filled 


with one state of matter. It is all now in what we call the gas or 
vapour phase. Other liquids show similar behaviour, each at its 
own particular temperature, and this temperature above which 
the substance can never appear in the liquid phase is known as the 
critical temperature. The pressure corresponding is called the 
critical pressure, and for water it is nearly 195 atmospheres, 
but however big the pressure no substance can appear in liquid 
form above its critical pressure. Incidentally this is the reason 
why all the earlier attempts to liquefy air, hydrogen and other 
so-called " permanent " gases failed, because it was not then 
recognised that the temperature must be lowered below their 
respective critical points before liquefaction could appear. 

Returning to our tube of water let us now cool it down. 
Exactly the reverse changes occur to those outlined above, till 
we come back to ordinary temperatures, by which time the 
pressure inside the tube is only about 15 mm. or, say, only 2 per 
cent, of one atmosphere. If we go on cooling below this, the 
vapour pressure steadily falls till at o C. the water begins to g;o 
solid by crystallisation and eventually only the ice phase is 
present. The pressure of the gas or vapour in the tube when 
ice and water are present is only 4-6 mm., and it remains at 
this till no liquid is left, showing that ice has an appreciable 
vapour pressure. If we go on cooling our tube to still lower 
temperatures the ice of course remains, but its vapour pressure 
steadily diminishes; nevertheless it is appreciable at quite low 
temperatures, and even at - 50 C. it is '03 mm. 

If we carry out a similar set of observations with water in an 
open glass vessel, such as a beaker, there is this important 
difference to bear in mind, that the pressure above the water 
is no longer its own vapour pressure i.e., a function of the 
temperature but constant. It is, in fact, about 760 mm., or 
the pressure of one atmosphere. If we cool the water down to 
freezing-point ice will begin to form at a very slightly lower 
temperature than it did in the tube, because of course 
the pressure is greater and the melting-point of ice is 
lowered by increasing the pressure. The effect is relatively 
small, but nevertheless the true constant temperature in the 
equilibrium, ice-water-steam, where all three phases coexist, 


is not quite the same as the temperature of an ice-water system 
under the higher pressure of one atmosphere, but a trifle higher. 
The true point, called the triple-point, is an absolute constant 
of nature, and its recognition by Willard Gibbs and later workers 
has led to a wonderful superstructure of theory known as the 
Phase Rule, by which the degrees of freedom of the varying 
phases of different forms of matter have been elucidated. The 
Phase Rule is much too complex to enter into here; suffice it to 
say that thisphysico-mathematical instrument has been of signal 
service, not only in the development of chemistry and geology 
in defining the limits of existence of crystals and rocks, but also 
in the successful exploitation of chemical discovery for large- 
scale industrial manufacture. Returning once more to water, 
let us heat our beaker containing it and enquire why it behaves 
differently from the water in the closed tube. For as everyone 
knows it evaporates constantly and when a certain temperature 
is reached the water boils. After what has been said it should 
be easy to understand why. On the kinetic theory the molecular 
velocity or agitation increases as the temperature rises, the 
velocity average of the molecules in the liquid phasebeingconstant 
for any temperature. At the surface of the liquid some of these 
molecules escape by bursting through the topmost layer and so 
appear above as gas molecules, and, conversely, the velocity of 
the molecules in the vapour space above will carry some of them 
back through the surface skin of liquid. In this battering of 
molecules to and fro there is thus a continual interchange going 
on between molecules on both sides of the surface ; and in a dosed 
tube when equilibrium is established for any particular tempera- 
ture there will be, on statistical average, as many escaping from 
the liquid to the gas-phase as vice versa. This equilibrium, 
however, changes when the temperature is raised and in such a 
manner that at the higher temperature a much greater number 
(concentration) are required in the gas-phase in order to com- 
pensate the greater number fired into this phase from below. 
This expression " fired from below " is crude, but it sufficiently 
meets the case, for the problem is really one of bombardment, 
and the point to remember is that for every temperature in an 
enclosed space there is an equilibrium in which an equal number 


of projectiles pass each way. This is equivalent to saying that 
the vapour pressure is always constant for any fixed temperature, 
and that it rises as the temperature rises because the speed and 
number of the projectiles are increased. 

Evaporation. We now see why the case is different in an 
open vessel. Evaporation is, of course, due simply to the steady 
loss of these projectiles, since in the open most of them get away 
into the air; only relatively few can return, not many as in 
the case of a closed tube. Evaporation will be more rapid at 
a higher temperature, for obvious reasons, than at a lower one, 
and most rapid for any temperature when a draught of air above 
the surface carries away the gaseous molecules of steam and so 
reduces the number at any instant which can return. For this 
reason although evaporation of water, which takes place slowly 
at o C. or even (more slowly) with ice and snow at much lower 
temperatures, can be fairly rapid in the open on a cold, windy 
day; and the rate of evaporation will be higher the higher the 
, wind and the drier the air, just as it will be lowest on a cold, 
damp, still day. 

Ebullition. Returning to our open beaker of water which 
we are heating, let us continue till the boiling-point is reached 
(100 C.). At the boiling-point the vapour pressure of the 
liquid just amounts to one atmosphere, and beyond this the 
pressure (in an open vessel) cannot go. So if more energy 
in the form of heat is supplied something has got to happen to 
the agitated molecules of the liquid ; and if the latter, as is usual, 
is heated from below, gas bubbles of steam collect and escape. 
The liquid is said to boil, and the boiling-point of any liquid is 
the temperature at which its vapour pressure just equals the 
super-incumbent pressure. 

But it must be emphasised that the liquid will not boil if 
maintained merely at its boiling-point. There is, in fact, very 
much more kinetic energy in a given mass of steam molecules 
than in the same amount of water at the same temperature. 
Before the water molecules can be endowed with this large 
extra amount of energy, it is necessary to supply it (latent heat 
of vaporisation see p. 125), as, for instance, is done usually by 
means of a fire or gas flame underneath, the energy of which is 


derived chemically by the combustion of carbon compounds. 
So water will boil quickly or slowly, say, in a kettle, according 
to the amount of energy supplied, this energy being transformed 
into the new form of kinetic energy of the steam molecules. 

Suppose the pressure above instead of being atmospheric is 
raised, say, tenfold, as it is in a closed steam boiler. The boiling- 
point is now much higher (180 C.), and of course the concentra- 
tion of the steam as well as its kinetic energy greatly increased; 
and so also its power of doing useful work in an engine. On 
the other hand, suppose the pressure is reduced below one atmo- 
sphere, as it is on mountain-tops; the boiling-point is now below 
100 C, and cooking watery food would accordingly be more 
difficult at high altitudes, because at the lower temperature the 
speed of the physical and chemical changes involved in cookery is 
lowered as the speed of the molecules (kinetic energy) is reduced. 
For example, it would be impossible to cook at all by boiling in 
open vessels on the planet Mars, because the atmospheric 
pressure (and therefore boiling-point) is too low. 

Distillation. In fact, the boiling-point of any liquid at any 
pressure is known if the vapour pressure figures are known, 
because the two are identical. So for example we know that a 
water boils at 80 C. at about half an atmosphere, at 60 C. fora 
quarter atmosphere, at 40 C. for one-tenth atmosphere, and at 
ordinary temperature at ordinary laboratory vacua (one-fiftieth 
atmosphere) . This principle of reduced boiling-point for reduced 
pressure is enormously used in industry, in what is called vacuum 
distillation. Ordinary distillation, which may be conducted 
on the small scale in a glass retort, or in a flask provided with 
suitable side-tube, consists in heating the liquid continually 
to make it boil, the vapour passing away continually through 
a cooled tube or coil, called a condenser, the function of which 
is to liquefy the vapour. The object of such distillation is to 
purify the liquid and separate it from accompanying substances, 
which are either not volatile at the boiling-point of the liquid 
distilled (say, salt dissolved in water) or have a higher boiling- 
point and tend to come over later in the distillation (this being 
called fractional distillation). But there are many substances 
which are rather unstable and tend to decompose on heating 


to their boiling-point at atmospheric pressure (glycerine, for 
example, which boils at 290 C.)- In such cases the practice is 
to resort to vacuum distillation, better described as distillation 
under reduced pressure (because a complete vacuum is never 
attainable), and so by working at much lower temperatures 
obtain all the separation-advantages without accompanying 

The Inert Gases of the Air. We have seen how not only the 
ancient classical writers but also the physicists and chemists 
of the period after the rebirth of science concerned themselves 
with the nature and properties of supposed elements, the four 
so-called " Aristotelian Elements ": fire, air, water and earth. 
We have also seen how, after a long period of misapprehension, 
fire or heat was proved by Rumford and Davy to be not an 
element in any sense but "a mode of motion." Water, 
owing to the labours of Cavendish and others, was shown to be 
not an element but a compound of two gases, hydrogen and 
oxygen, united in certain fixed proportions (pp. 143, 174). 

Air was the chief " element " to receive attention, and here 
again a composite nature was revealed by Mayow, Black, 
Scheele, Priestley, Cavendish and others, who showed it to be a 
mixture of gases of which the chief were the elements nitrogen 
and oxygen. It will be remembered, however, that Cavendish, 
in 1784, tried to determine the exact nature of the gas left over 
after he had caused all the oxygen in the atmosphere to unite 
with hydrogen to form water, and found that about -fa of 
the residual nitrogen was something that was not "mephitic 
air " or nitrogen (p. 175). No notice was taken till the nature 
of this " something " was discovered about a century afterwards 
by Rayleigh and Ramsay. 

In 1893 Lord Rayleigh, at that time Professor of Physics at 
the Royal Institution, was occupied in determining the densities 
of various gases, and observed that "pure" nitrogen got from 
the air was always slightly heavier than the same gas when 
obtained chemically from compounds of nitrogen. He there- 
upon associated with himself Sir W. Ramsay, Professor of 
Chemistry at University College, London; and, using all the 
best modern methods and appliances, the two scientists at last 


were successful in showing that the additional weight was due 
to the admixture in small amount of a new gaseous element, 
which they christened " argon/' " the lazy one/' because it 
could not be induced to combine with any other element. 

In 1895 Ramsay made his famous discovery of helium in 
the mineral cleveite, the gas that had been found, many years 
before, by Lockyer to form a constituent of the solar chromo- 
sphere (p. 164). In 1898 Ramsay and Travers prepared large 
quantities of crude atmospheric argon, and, by liquefying it with 
intense pressure and cold, followed by repeated distillations, they 
obtained not only helium but three other new gases, neon, " the 
new one/' krypton, " the hidden one/' and xenon, " the stranger/" 
not one of which would form a compound with any other element, 
and so they all were named inert gases. Taken together these 
five gases constitute about i per cent, of the atmosphere, so 
that Cavendish, with his somewhat primitive apparatus, was not 
far wrong in his estimate of the amount of the strange gas present 
in atmospheric nitrogen. It may be said, therefore, that we 
are now well acquainted with the nature and composition of the 
gaseous envelope of our globe (see also p. 365). 


Cathode and Anode Rays. In 1880 Sir William Crookes. 
made a discovery that led to new methods of investigating^ 
the constitution of matter. He was studying the effects pro- 
duced by sending electric discharges through tubes from which 
as much as possible of the contained air had been extracted 
" vacuum tubes ' ' as they were called. It had long been known 
that an electric current could not be induced to pass through 
gases save at a very high voltage. Crookes tried whether the 
discharge would take place in more and more rarefied pressures, 
and obtained some very remarkable results. 

He carried out his experiments in tubes similar to those seen 
in Fig. 91. When, by means of an air-pump at first attached at 
P, before the tube was hermetically sealed, the air pressure 
within was greatly reduced, the gas became luminous. At 
the cathode end (i, K) there appeared a dark space, called 
the "Crookes dark space" (CK) followed by a glow (K G), 


and then another dark space, the "Faraday dark space" 
(F), succeeded by wave-like extensions toward the anode 
(A), known as the " positive column " (P C). When the gas 
pressure in the tube was reduced to o*or mm. of mercury, 
the positive column disappeared, and the " Crookes dark 
space" filled the tube, the end of which began to glow. 
These kathode (cathode) rays which cause all these phenomena 
make mica vanes within revolve like the arms of a windmill ; they 
can be deflected by the north pole of a magnet, and there- 
fore must carry charges of negative electricity, as may be seen 
by projecting them into a cup placed in connection with a 

galvanometer (Fig. 91, 3, G). 
They may be intercepted 
by some article, and cast a 
shadow of it on the wall of 
the tube (4). What are these 
particles ? Crookes regarded 
them as a "fourth state of 
matter." They are now 
identified with "electrons" 
the ultimate and indivis- 
ible but material units of 
negative electricity. 

If the cathode be pierced 

FIG. 91. VACUUM TUBES AND CATHODE ^^ j^^e holes, another 

set of rays may be recog- 
nised travelling from the anode in the reverse direction towards 
the cathode pole; and these rays, under powerful magnetic 
influence, may be deflected also, but in a direction opposite 
to the cathode rays, showing them to be positive and with 
a mass proportional to the atomic weight of the gas in the 
tube. They are generally regarded as the atoms of the resid^ 
gas left in the tube that have lost one or more electrons/ If 
an absolutely perfect vacuum could be obtained in the tube 
there would be no anode rays, but such a vacuum as yet is 

Rontgen or X-Rays. In 1895 Rontgen, professor of physics 
at Wiirzburg, made a further discovery in relation to cathode 


rays. He found that when they impinged on a metal plate 
they gave rise to a new form of radiation which, because he 
was unable to determine their exact nature, he called " X " 
or unknown rays. The important point about them was that 
they had immense penetrating power, a power that increased 
the more perfect the vacuum in the tube. Looking back to 
the chart of the spectrum figured on p. 107, it will be seen that 
X-rays range from 0*000,000,001 cm. to 0*000,001 cm. as con- 
trasted with the wave-lengths that make impressions on the 
retina of the eye, which range from violet, 0*000,038 cm., to 
red, 0^000,078 cm. Fig. 100 shows the usual form of an X-ray 
tube. The cathode is a concave plate by means of which the 
electrons are focussed on a 
flat metal mirror, B, from 
which the X-rays emanate 
through the wall of the bulb. 
If aphotographic plate, P, en- 
closed in a light-proof cover- 
ing, be placed on a table, 
and an ob j ect , say the human 

hand, be laid on it, the P - '. . _ *> ... 

rays penetrate the hand un- ^ 92 ._ X -R^ APPARATUS. 
equally. The flesh, being 

much less opaque than the bones, is penetrated by the rays, which 
are retarded or stopped by the bones. These latter, therefore, 
produce dark shadows on the plate. The result is a photographic 
negative, from which " radiographs " may be printed, enabling 
a surgeon to see the exact nature of a dislocation or a fracture 
in a bone, or the spot where a foreign body, a needle or a bullet, 
has lodged, as if the whole of the flesh had been dissected away. 
Electrons, thus discovered by Crookes and used by Rontgen to 
excite X-rays, by bombardment of metals, were destined to play 
a big part in twentieth century science, as we shall now see, 

The Constitution of Matter and the Structure of the Atom. 
The reader is advised to frequently consult the table on 
pp. 380, 381, while reading the rather difficult pages which now 
follow. Before dealing with modern views, it will be as well 
jto review the Atomic Theory as it stood at the close of the 



nineteenth century, after it had surmounted the early difficulties 
of the first half of the century, arising from the confusion 
between atoms and molecules. These grave difficulties were 
cleared away in 1858 by the penetrating genius of Cannizaro, 
who showed what the true consequences were of Avogadro's 
hypothesis of 1811 (p. 183). This hypothesis, which enables 
.the molecular weight to be deduced of aU pure substances 
that can appear in gaseous form (say, by heating), is fundamental 
to chemistry. For if the weight of the molecule is known in 
terms of the weight of an atom of hydrogen as unity, the 
weights of the atoms composing the molecule can be deduced 
from the combining weights, but not otherwise. We know, 
for instance, that the molecule of water is H 2 0, if that of 
hydrogen is H 2 , because (i) the density of steam is nine times 
that of hydrogen (taken as unity) and (2) because 8 parts 
by weight of oxygen are present to i of hydrogen. For since 
equal volumes contain the same number of molecules (Avogadro) 
the water molecule must weigh nine times as much as a hydrogen 
molecule and therefore eighteen times as much as a hydrogen 
atom. So in this water molecule there must be i atom of oxygen 
weighing 16 and 2 hydrogen atoms, each weighing i. All this 
rests on the belief that there are 2 hydrogen atoms in a molecule 
of hydrogen, which is independently proved by the fact that 
2 volumes (equivalent to 2 molecules) of hydrogen combine 
with i volume (equivalent to i molecule) of oxygen, producing 
2 volumes (equivalent to 2 molecules) of steam. A little logical 
thought here wiU show that you could not get 2 molecules of 
steam, of density 9, from 2 molecules of hydrogen if the latter 
consisted of 2 single atoms which cannot be split, but that 
these molecules must contain at least 2 atoms each. This will 
perhaps be dearer if we write the equation thus: 

2H 2 + 2 =* 2H 2 

ti mols., therefore 2 volsA fi mol. t..,'i vol\ / moh i.e., a volsA 

( density i J V density 16 / V density 9 ; 

On this line of reasoning the whole vast superstructure of 
chemistry has been erected, and the chemical formulas of 
thousands of compounds deduced. When it is recollected 
that organic chemistry, with its hundreds of thousands of 


compounds of known constitution, has been "built up on the 
molecular theory founded on Dalton's atoms, without a single 
exception being discovered inconsistent with this theory, it is 
self-evident that the theory must be fundamentally sound; 
and that atoms, each with its own definite combining weight, 
have a definite objective reality and are not a mere convenient 
figment of the imagination. In the almost romantic develop- 
ment of organic chemistry during the latter half of the nineteenth 
century, the atoms played the part of units or bricks in molecular 
architecture, and the great quest was (and still is) to build these 
bricks into every imaginable type of structure (synthesis) or 
find out how they were linked together in known substances 
(naturally occurring organic compounds associated with life). 
In this great and wonderful achievement chemists did not 
bother their heads with the bricks themselves or what they 
were made of; and up to the end of the nineteenth century 
atoms were still regarded in much the same kind of way as 
Dalton had imagined them. Indivisible units they were to the 
chemist because, so far as could be then judged, no power could 
split them up into anything simpler. And even if they could, 
this would not affect the validity of the great truths of chemistry 
which had been established, and indeed which still hold. It 
was, of course, recognised that they might not be the ultimate 
stuff of the Universe, any more than a brick, which is composed 
of particles of clay, is an ultimate material unit. Prout's 
hypothesis (p. 185) had been discountenanced by the accurate 
atomic weight determinations of Stas (p. 252) and others, and 
mere speculation as to the constitution of atoms led nowhere. 
So that until a generation ago the generally received concep- 
tion of the constitution of matter was that, on ultimate analysis, 
it consisted of invisible, indivisible and indestructible particles 
or atoms which Lord Kelvin, in 1881, estimated to be anything 
between a ten-millionth and a hundred-millionth of a centimetre 
in diameter. But as we learnt more and more of the nature of 
electricity, doubts began to arise as to the indivisibility of the 
atom. In the closing years of the nineteenth century, Sir 
J. J. Thomson, of the University of Cambridge, suggested that 
an atom consisted of two parts, one electrically positive, the 


other electricaUy negative. The positive charge resided in a 
" nucleus/' and the negative charge or charges in one or more 
" electrons " which surrounded the nucleus. This may be 
called the " static " theory of the atom. The electron was 
estimated by Thomson to be about y^Vrr of the mass of a 
hydrogen atom, and, since hydrogen had only one electron, 
almost the whole of the mass of the atom lay in the nucleus. 

Rutherford-Bohr Atom. These tentative views received 
striking confirmation when in the early years of this century 
the mysteries of radioactivity (p. 273) came to be slowly 
unravelled. It was more particularly the behaviour of radium 
and uranium which forced the new views of atomic constitution 
on the minds of physicists, and indeed a new world of science 
was opening up the like of which could never have been imagined 
by the chemists of the nineteenth century. Bit by bit the 
entrancing story was unfolded, more like a tale from fairyland 
than the supposedly dry findings of science, but alas 1 complex 
beyond measure and completely intelligible only to the trained 
mind. We shall not give these findings piecemeal, as they 
became accepted, but attempt a very simple description of the 
present-day theory of the constitution of the atom. 

A tremendous advance took place when in 1911 Sir E. 
Rutherford put forward the view, extended by Niels Bohr, that 
an atom might be simply a miniature solar system, consisting 
of a central sun the nucleus and one or more planets the 
electrons whirling round it. This is the basis of the 
planetary atom or "dynamical' 1 theory, which is now 
generally accepted. 

In the Rutherford-Bohr atom it is necessary to picture a state 
of affairs (diagrammatically indicated on p. 246, Fig. 93) in which 
an excessively minute central nucleus is surrounded by one or 
more electrons at a relatively enormous distance, these electrons 
moving round the nucleus at prodigious speeds. Practically 
the whole mass of the atom (i.e. 9 its atomic weight) resides 
in the nucleus, the revolving electrons being so light that they 
contribute almost nothing to the weight. To grasp the mag- 
nitudes involved it is best to consider the hydrogen atom model 
(i) , this being the simplest of all atoms. Imagine then a central 


nucleus, the so-called proton, surrounded by one moving electron. 
The proton weighs 1,840 times the electron, but its actual mass 
is so small that it would take about a million million million 
million protons to weigh I gram. Its size is so inconceivably 
minute that it would take 5,000 million million protons in a 
contiguous string to give the length of one centimetre. The 
electron while much lighter is thousands of times larger than 
proton. These incredibly small magnitudes have been deter- 
mined experimentally and are known with a fair degree of 
precision, and if they convey anything to the human mind 
it is the vast emptiness of the atom; for when the distance of 
the electron from the proton is taken into account, it is found 
that in the normal (un-agitated) hydrogen atom this distance is 
about 25 million times the diameter of the nucleus (proton) 
and very much greater still when the hydrogen atom is endowed 
with increased energy, such as is associated with the high 
temperature of the stars. The size of the normal (undisturbed) 
hydrogen atom is the size of the orbit which its single electron 
describes round the proton, and this is such, that it would take 
a contiguous string of 100 million atoms to measure i centimetre. 
Some conception of the infinitesimally small magnitudes in 
question may be obtained by remembering that one million 
days ago, from today, would carry us back to 900 B.C. that is, 
to a period before the rise of Greek civilisation. Whilst the 
actual figures representing magnitudes convey but little to the 
imagination, the model of the atom representing relative 
relationships is intelligible enough, and we may proceed to con- 
sider the simplest such model (hydrogen) further. This model 
is shown in rough and not to scale in Fig. 93, where the dark 
inner circle, numbered i, crudely represents the proton-nucleus 
and the larger circle, with dot, the orbit of the electron. This 
relatively massive proton has been identified with the smallest 
known unit of positive electricity (marked + in the diagram), 
and the light electron has similarly been shown to be the smallest 
unit of negative electricity. Yet each are particles of matter in 
the sense that they possess mass and are subject to gravitation; 
and so in its last analysis matter (in the form of protons and 
electrons) is indistinguishable from electricity. This, as we 


shall see presently, applies to all forms of matter at present 
we are only considering hydrogen. 

The electrical attraction between the proton, carrying one 
charge of positive electricity, and the electron, carrying one 
charge of negative electricity, is such that if they could come 
together both would disappear in a flash of radiation, the 
amount of which has been calculated and found to correspond 
in quantity to that of the cosmic rays referred to on p. 269. If ? 
as there is reason to suppose, this can happen (in the^ stars 
and nebula) it involves the annihilation of matter and its re- 
placement by an equivalent amount of energy. So far as we 



know, however, under conditions obtaining on earth this does 
not happen to any appreciable extent. The electron, situated 
as it is at a relatively enormous distance from the proton, does 
not fall to it any more than the planets fall into the sun; it 
revolves round it instead, and this also is true of other atoms 
where there are heavier nuclei than a proton and two or more 
electrons. The latter revolve rapidly around the nucleus in 
definite orbits, and it is only this high speed of revolution which 
prevents the universal disappearance of matter in a stupendous 
outburst of energy. Moreover, it is this motion of electrons, 
at relatively vast distances from their nuclei, which confers 
upon the atoms of all elements their stability and specific 


feature of indivisibility. So also it comes about that the size 
of any atom is vast compared to the size of the central nucleus, 
and that an atom is enormously bulky compared to the weight, 
nearly all of which is centred in the nucleus. 

Returning to the model hydrogen atom, Bohr has worked out, 
by an intricate analysis, what the movements of the electron 
are under various conditions of stimulation, by tracing out the 
relation between the spectral lines of hydrogen under varying 
conditions of excitation. He has established clearly that the 
electron can have many orbits, but that these orbits are related 
to each other in a very simple manner. Thus they may be 
circles whose diameters stand in the ratio to each other i 2 , 2 2 , 
3 2 , 4,2 5 2 , etc. i.e., i, 4, 9, 16, 25, etc., where unity, i, stands 
for the diameter, already referred to, of the orbit in the hydrogen 
atom in its un-excited (cold) condition. Also the orbits may 
be ellipses of different eccentricities, but the major axes of 
these elliptical orbits stand in these same ratios, i, 4, 9, 16, etc. 
Intermediate orbits in no case appear. When the atom is 
excited to a greater activity, say by raising the temperature, 
the electron jumps from an inner lower orbit (say 4) to a Mgher 
one (say 9), whether circular or elliptic; and in doing so a very 
definite amount of radiant energy is absorbed, called a quantum. 
When an atom in an excited condition changes to one of lower 
energy, the electron jumps from this outer orbit to an inner one 
(say from 9 to 4), whether circular or elliptic, and in doing so 
gives out the same quantum as it absorbed in changing from 
4 to 9 viz., as a definite line in the spectrum of radiation. 
We will explain the nature of radiation more fully later on 
(pp. 257-273) ; for the present it is sufficient to say that the 
emission of energy is detected spectroscopically, and that the 
precise orbital changes or electron-jumps can be correlated with 
definite spectral lines. 

Let us now turn to other atoms. All matter, as we know 
it on earth, is made up of combinations or mixtures of the 
92 elements given in the table on pp. 380, 381. These elements 
are numbered in a sequence from i to 92, for a reason which 
will appear presently, these being the atomic numbers; and of 
the whole sequence only two (85 and 87) are as yet undiscovered. 


Each of these elements differs from the others, and there is a 
steadily increasing atomic weight as we pass up the series. 
How does this come about, and what is the constitution of these 
92 kinds of atoms ? 

The Rutherford-Bohr model explains them very simply. 
Each atom is similar to hydrogen in that it possesses a positively 
charged massive nucleus, inconceivably small, in which practi- 
cally the whole weight of the atom resides; and around this 
nucleus are electrons circling in definite orbits, the number of 
these orbital electrons being different for each element. Take, 
for example, the next simplest atom to that of hydrogen, 
helium, of atomic number 2 (see Fig. 93, 2). The nucleus has a 
mass rather less than that of 4 protons, but it also has 2 electrons 
in some way embedded in it. As it is believed that there are 
actually 4 protons (i.e., 4 positive charges) present, 2 of which 
are neutralised by these embedded electrons, the net charge 
on the nucleus is 4-2 i.e., 2 positive charges. The latter 
therefore require 2 negative charges (as orbital electrons) to 
make the neutral atom of helium. Although in the diagram 
these 2 electrons are represented as lying in the same ring, as 
a matter of fact they pursue independent quantum-orbits 
and do not chase each other in the same orbit as might appear 
from the figure, which is to be regarded as purely diagrammatic. 
These independent orbits, circular or elliptic, may vary greatly 
in size but always have diameters related to that of the inner- 
most, in the ratio I, 4, 9, 16, etc., just as with hydrogen; 
and the electron-jumps from any outer to an inner orbit 
correspond to a definite emission of energy, the quanta giving, 
as might be expected, a rather more complex spectrum than 
that of hydrogen. 

The next element, lithium (atomic number 3), has seven 
positive protons bound with four electrons in the nucleus, which 
thus has a net positive charge (7-4) of 3, neutralised by 
three orbital electrons; and so on, until we reach the 
weightiest element, uranium. Here we have a nucleus of 238 
protons with 146 nuclear electrons and a net positive charge 
therefore of 92 (238-146), thus requiring ninety-two orbital 
electrons* The number of free (orbital] electrons in all cases gives 


the atomic number of the element. The sequence of the atomic 
numbers of the elements was determined by the brilliant young 
physicist, Moseley (who lost his life in the Gallipoli campaign), 
by using X-rays in his researches (see Table, pp. 380, 381). 

We have now got to the length of believing that the electron, 
the ultimate unit of negative electricity, when combined with 
a unit of positive electricity, forms matter (hydrogen), but what 
the unit of positive electricity (the proton) is we have as yet no 
more conception than we have of what the electron is. In an 
after-dinner speech one of our distinguished physicists, when 
asked to explain the difference between electricity and matter, 
replied that it was "immaterial/' reminding us of the older 
definitions of mind and matter attributed to T. H. Key, once 
Head of University College School, "What is mind? No 
matter. What is matter? Never mind/' 

Electron Rings. It is necessary now to consider how these 
orbital electrons are distributed in the atoms of the various 
elements, according to Bohr's theory, and how some of them 
may be detached; though it will not be possible, in a simple 
description such as is outlined here, to give the underlying 
physical and mathematical reasons for the distribution assigned. 
To simplify matters we will consider neutral atoms (i.e., those 
with their full complement of orbital electrons) and leave out 
all questions of movements of the various electrons in their 
many circular and elliptical orbits. To this end we can con- 
veniently regard the electrons as stationary. It is found that, 
as the atomic numbers of elements increase from i to 92, the 
electrons distribute themselves in a very definite manner, in 
the form of successive rings or shells surrounding the nucleus, as 
it increases in the elements of increasing atomic number from 
a mass (atomic weight) about i to about 238. Whilst the atom 
of helium has a simple ring of 2 electrons, the next higher atom, 
lithium (atomic number 3), has not got its 3 electrons in a 
similar ring of 3; it has 2 electrons like helium in an inner 
shell, while the third occupies an outer position or imaginary 
ring which is quite distinct . Moreover, this electron is relatively 
easily detached from the atom of lithium, and in this property 
of detachability it confers upon the element lithium many of its 


characteristic chemical properties (ionisation). When we pass 
to the next higher element (beryllium), whose nucleus weighs 
about nine times as much as a proton and has 4 surrounding 
electrons (atomic number 4), we find again that only 2 are in the 
inner ring, while the other 2, which are more mobile, occupy the 
same kind of outer ring as that occupied by the outer lithium 
electron. And so on as we pass up the series of atomic numbers 
with increasing weight of nucleus, each additional electron 
finds a position in this outer or second ring, the inner one 
never having more than 2 electrons. Thus, taking the ascend- 
ing series in succession, the next element , boron, has 3 such outer 
electrons, carbon 4, nitrogen 5, oxygen 6, fluorine 7, and neon 8. 
Up to fluorine, one or more of these outer-ring electrons are 
easily detachable from the atoms (conferring characteristic 
chemical properties by so doing), whilst the inner ring of 2 is 
stable and unaffected by chemical treatment. 

Neon has atomic number 10 with a nucleus weighing about 
20 protons, and when we pass to the next higher element we 
notice something very remarkable, which reminds us of the 
passage from helium to lithium. Sodium (atomic number n) 
is this next element, but it is found that the extra electron it 
contains (as compared to neon) does not slip into the same 
second ring as with the previous series, but starts in a new outer 
ring of its own. This makes a total of 3 rings now namely, the 
inner ring of 2, the second ring of 8, and the outer ring of i. 
And as we continue passing up the series of elements by in- 
creasing the weight of the nucleus and adding one electron at a 
time, we find that " history repeats itself/' so to speak; these 
additional electrons range themselves in this third ring, one 
by one, as we increase the atomic number step by step, and as 
before one or more of these outer ring-electrons are easily 
detachable and responsible for the chemical properties of the 
respective elements. But when a second octet ring (8) is com- 
pleted the filling-in process stops a ninth electron cannot be 
inserted in this third ring, any more than it could before with 
the second ring. 

The atom whose third ring has 8 electrons is that of the 
element argon, and it is a most significant thing that helium, 


neon, and argon, each, of which has completed shells or rings 
of electrons (respectively 2, 8, and 8), that refuse to accept 
any more, are remarkably similar in being inert gases that is 
to say, unlike the other elements, they refuse to enter into 
chemical combination with anything. 

It would seem then as if chemical reactivity requires an un- 
completed outer ring of more or less easily detachable electrons, 
and for this reason the outer rings of elements are called valency- 
rings, valency being, as we shall see later (p. 379), an expression 
of the saturation-capacity of an atom by other atoms, on 
chemical combination. 

As we proceed higher up the series of elements, and the 
nucleus increases in weight, the case becomes more complicated 
and need not be followed here, except to say that with the 
formation of further outer rings, the completed shells or rings 
may have 18 and not 8 electrons. 

So distinct and so fundamental to physics and chemistry 
are these successive electron-rings of the elements that they 
have been given the labels K-, L-, M-, N-, etc., to distinguish 
them. The K-ring is the innermost one of 2 electrons; the 
L-ring is the next outer one of 8 electrons, the M-ring the next 
outer one of 8 electrons, and so on. We speak, for instance, of 
light-radiation disturbing, say, the N-ring, of X-rays disturbing 
the K-ring, and of chemical reactions or high temperature dis- 
turbing the outermost ring, whichever it happens to be. 
Although this is perfectly easy to understand in terms of a simple 
static model, such as we have for the moment been considering, 
the problem becomes bewilderingly complex when account is 
taken of the high-speed movements in circular and elliptic 
orbits, each related to that of lowest energy by the respective 
diameters, i, 4, 9, 16, etc. ; especially when it is remembered 
that no two electrons can occupy the same orbit any more 
than two men can stand on the same rung of a ladder. 

We can look at it something like this, taking the neon atom 
to illustrate: Its outer octet L-ring, when actual electronic 
movements are taken into account, resolves itself into 8 indepen- 
dent orbits of lowest energy for the un-excited atom, and its 
K-ring similarly resolves itself into 2 independent orbits of 


still lower energy. Each of these independent 10 orbits, in their 
respective distributions of 2 and 8, may now, on exciting the 
atom, change to larger orbits (circular and elliptic) whose 
diameters are related to the orbits of lowest energy by the 
sequence i, 4, 9, 16, etc. When any single electron drops back 
from any of the higher to any of the lower orbits, a single 
quantum of energy is emitted as radiation, but the whole radia- 
tion for the various quanta must necessarily be very compli- 
cated. This complication reveals itself in the spectral lines 
which enable the actual electron-jumps to be elucidated. 

If the case is complex for an atom like neon containing 
only 2 rings (K and L), it will be readily understood that it 
is so complex for atoms of higher atomic number as to baffle 
complete disentanglement. Moreover, difficulties with the 
Bohr model have appeared in recent years, which are dealt 
with on p. 272. 

Atomic Nuclei. Leaving aside for the moment the orbital 
electrons of atoms, let us now consider more fully the nature 
of the nuclei, in which virtually the whole of the weights of 
atoms (atomic weights) resides. It has already been noted 
that these weights are approximately multiples of the weight 
of one proton, and this would seem to suggest that, just as Prout 
suggested long ago (1815), all atoms of matter are ultimately 
made up of hydrogen, whose atom consists of one proton. 
As we shall see later (p. 283), there is experimental evidence 
for this view as well as good theoretical grounds, and it only 
remains to explain why the atomic weights of all element s are not 
exact multiples of that of hydrogen (i) if the atoms of all matter 
are contrived out of protons, in steadily increasing numbers, 
by packing them together in the nuclei of these atoms. After 
the middle of last century Stas made a series of most extra- 
ordinarily accurate atomic weight determinations, in the hope 
of proving or disproving a whole number rule, showing the 
truth or otherwise of Prout 's hypothesis. He clearly indicated 
that, while many elements approximated closely to a whole- 
number atomic weight if oxygen were taken as the standard 
at exactly 16, which means hydrogen would be 1*008 instead 
of exactly i, there were other elements like chlorine which were 


hopelessly out of it and nowhere near a whole number; and so 
Prout's hypothesis was abandoned. 

But we now know that chlorine and the other elements 
whose atomic weights are not whole numbers are not " pure " 
elements. They are chance mixtures of two practically identical 
elements of different atomic weight, which are to all intents 
and purposes whole numbers on the basis of = 16. Such 
" mixed " elements have nuclei of different masses, but these 
elements have the same chemical properties, so are virtually 
indistinguishable and inseparable from one another; and they 
are called isotopes (see p. 256). Many such isotopes are now 
known, and their discovery put an entirely different complexion 
on the problem, for it turned out that in every case each indi- 
vidual of any isotopic mixture had without exception an atomic 
weight which was practically a whole number, on the basis 
= 16. Practically, but not quite. And now the small dis- 
crepancy, as well as the reason why the standard cannot be 
taken on the basis of H = i, was at last run to earth. It is that 
there is a definite loss of mass, referred to by Aston under 
the name " packing fraction," when several protons are bound 
together with some electrons into the nucleus of an atom. The * 
loss is greatest when 4 protons are packed together with 2 
electrons to give the helium nucleus, as explained on p. 382, 

So now Science has reached the rather startling conclusion, 
though in modified form, that Prout was right and that all 
matter of the Universe is ultimately made up of protons i.e., 
of the same stuff as hydrogen. It comes to this then, that the 
nucleus of any other atom is made up of a definite number of 
protons (each of unit weight on the basis O = 16) somehow bound 
together with a certain number of electrons. And the difference 
between the number of protons (each of which is an atom of 
positive electricity) and the number of binding-electrons (each 
of which is an atom of negative electricity), represents the net 
charge of the nucleus. This net electrical charge is always posi- 
tive, because in every nucleus there are always more protons 
than electrons. 

Let us illustrate by taking a concrete case, say sodium, for 
the sake of simplicity, because it is one of the " pure " elements. 


The nucleus of the sodium atom contains 23 protons packed 
together with 12 electrons, which can be regarded as in some 
way cementing the whole structure together. The net positive 
charge on the nucleus is therefore n (i.e., 23-12), and so 
the nucleus requires n orbital electrons (i.e., n negative 
charges) to make the neutral atom. And the researches of 
Rutherford, as we shall see later (p. 281), indicate that the 
nucleus itself, of heavy atoms, has an orbital type of structure, 
in which protons probably revolve around a dense centre 
containing protons and electrons. 

lonisation. We have seen (p. 250) that some of the outer- 
ring electrons of an atom may be more or less easily detached, 
provided that this outer ring is not one of the completed series, 
2, 8, 8, etc., as in helium or neon. We may now enquire further 
into this phenomenon, which is known as ionisation and is 
very important, inasmuch as most chemical reactivity seems to 
depend on it. An atom which has lost one such electron has a 
single positive charge, like lithium, written Li+ ; one which 
has lost two has two positive charges, like Ca+ + , and so on. 
How are these electrons lost, and what are the ionised atoms 
like ? The principal agency in detaching outer electrons, so 
far as Nature is concerned, is simply thermal energy, and in the 
sun or stars more and more electrons are detached as the tem- 
perature rises, from surface to interior. Some atoms (like 
calcium) are more easily ionised in this way than others, and 
as the spectra of elements depend to a large extent on the 
degree of ionisation, it will be seen that stellar spectra are a 
fairly accurate measure of surface temperatures of the stars and 
sun. At the ordinary temperatures prevailing on earth there 
is no such thermal ionisation; the atoms are not only complete 
with their full complement of electrons, but these atoms mostly 
combine with each other (or other atoms) to form the molecules 
of familiar matter. And in doing so it is evident that the outer 
ring or valency electrons must be in some way concerned, 
because it is only those atoms, whose outer electrons are easily 
detachable, that can enter into these combinations which we call 
chemical reaction (so helium and the other inert gases cannot 
do so because of their complete and stable rings, and so it 


comes about that their atoms are the same as their mole- 

Nevertheless it is possible to detach electrons, under con- 
ditions obtaining on earth, by utilising any appropriate form 
of energy of sufficient potential. For instance, bombardment 
by other atoms (as we shall see, p. 277) will chip off electrons 
and ionise gases. Or, again, the rays of the sun striking mole- 
cules of oxygen and nitrogen in the air does a similar thing, 
knocking off electrons from the atoms in the molecules, which 
thus become ionised (positively charged), and the electrons may 
remain free or become attached to other molecules which become 
negatively charged. At night time when there is no sun, more or 
less neutralisation of these charged molecules occurs by the elect- 
rons being captured by atoms of molecules which had lost them. 

Again, mere rubbing of the molecular surfaces of matter 
may detach electrons from the atoms of the molecules, leaving 
an excess of them on one of the rubbed surfaces, which thus 
becomes negatively charged, and a deficiency on the other 
surface, which becomes positively charged. This, of course, is 
the explanation of frictional or so-called " static " electricity 
(p. 134), and when a spark or lightning passes between objects 
of opposite charge it is due to a surge of electrons passing from 
the negatively charged to the positively charged object; whilst 
an electric current consists of a continuous flow of electrons 
(from - to +) along such materials as metals which pass them 
on freely from atom to atom (conductors). Thunderstorms, 
considered later (see p. 367), arise from the frictional detach- 
ment of electrons from the molecules of water-drops (which 
become +) by a rapid air current, the molecules of which be- 
come negatively charged. 

Long before these things were clear and before J. J. Thomson 
had shown that the electron is the ubiquitous atom of negative 
electricity, chemists had been forced to the belief that certain 
types of compound, known as electrolytes, suffered dissociation 
into their ions ( + and - ) when dissolved in water. Electrolytes 
in solution conduct electricity, not like metals unchanged, but 
in such a way that a positive ion is attracted to the negative 
electrode or pc le (cathode) and a negative ion to the positive 


electrode (anode) ; and, as we have seen (p. 157), ** was Faraday 
who first made this great and fundamental discovery. The prin- 
cipal electrolytes are salts and acids, and, as we shall see later, 
they conduct electricity because in solution these substances 
do not merely dissolve unchanged like sugar, but actually break 
up into oppositely charged ions. When for instance common salt, 
NaCl, is dissolved in water the sodium atom parts with an elect- 
ron of its outer valency ring, becoming the positively charged 
sodium ion (Na+), whilst the chlorine atom picks up this electron 
and becomes the negatively charged chlorine ion (C1-). It is 
important to observe that the ions have quite different chemical 
characters from those of the neutral atoms; and, moreover, 
the chemical properties of the elements, as such, are largely due 
to the ease or difficulty with which they lose one or more valency 
electrons, passing into ions whose properties are quite different 
from those of the elements themselves. We will deal more 
fully with these matters when we come to consider the con- 
sequences of the theory of electrolytic dissociation (p. 378). 

Isotopes. We now return to isotopic elements. In 
Fig. 93, 6, potassium is represented as having 19 free elect- 
rons, giving the atomic number of 19, but, in 1920, Aston 
discovered that potassium had two kinds of atoms, one with 
the atomic weight 39 and the other with the atomic weight 41. 
Since the number of orbital electrons is the same in both forms of 
the element viz., 19 it follows that the nucleus of one atom 
has 39 protons and 20 bound electrons, while the nucleus of the 
other has 41 protons and 22 bound electrons. It is important 
to observe that the total number of electrons i.e., those bound 
in the nucleus plus those freely moving in orbits in any atom 
is equal to the total number of protons in the nucleus, and 
therefore is equal to the atomic mass (mass number) . The latter 
is the same as the atomic weight in the case of " pure " elements 
like sodium, but not so in the case of potassium, some of whose 
atoms (the majority) have mass 39 and some (a minority) have 
mass 41. So it comes about that the element potassium, as 
presented to us by Nature, has an atomic weight between these 
figures viz., 39*1. Many other elements e.g., lead (p. 354) 
similarly are made up of two or more kinds of atoms differing 


In weight, and to these the name isotopes (equal places) has been 
given, indicating that, though their atomic mass may differ, 
their chemical characters remain identical and they have the 
same atomic number (see p. 249). 

To conclude, we may say that the modern explanation of 
the chemical affinity of atoms, in forming compounds, involves 
a redistribution of the outer-shell electrons of the combining 
elements. These outer electrons are the valency electrons, and 
Langmuir showed that there is a tendency for themto form groups 
of eight (octet theory) round thecentral atoms after combination* 
All recent work goes to demonstrate that the fundamental 
properties of an element are determined by (i) its valency 
electrons; (2) its total orbital electrons. The number of the 
latter is the atomic number of the element, and it is the 
atomic number rather than the atomic weight which is 
significant in the identity of an element. These free or orbital 
electrons increase from i (hydrogen) through a steady sequence 
to 92 (uranium), the element of highest atomic number i.e., 
while many isotopes appear no higher elements are known. 

Radiation and the Quantum Theory. 

The advances made in recent years into the fundamental 
nature of radiation have so utterly transformed the outlook of 
the physicist and philosopher since the so-called "classical" 
science of the nineteenth century, that some attempt must 
be made here to explain the new views which are so pregnant 
in meaning to the cosmogony of today. The simple diagram 
on p. 107 will help the reader to follow this explanation. 

Anyone who warms himself before a glowing fire, or basks 
in the sun, is conscious through his senses of something (heat and 
light) which is conveyed across intervening space from the 
radiating object to his skin; that is to say, from a material 
which is radiating aero ss nothing material (for air is not necessary 
but rather a hindrance) to some distant material (or perchance 
nothing). Such radiation (heat or light) is a mobile form of 
energy, for it can do work and can be measured accurately in 
terms of work. But other forms of radiation, also energy, not 
perceptible to the senses can be detected and measured by 


scientific instruments, and a whole range of radiation has thus 
been disclosed, of which heat and light constitute a mere fraction. 
What is this radiant energy of such enormous range, and how 
does it travel across empty space, as, for example, from the 
sun, stars, and even more distant nebula ? No all-satisfying 
answer has yet been found to these simple questions. We 
know that the rays from any radiating object spread out in 
all directions in virtually straight lines (in absence of material 
influence), and so their intensity must fall off with distance 
according to the law of inverse squares. For example, Venus is 
about 7 times the earth's distance (taken as unity) from the sun ; 
it will receive, therefore, about twice the radiant light and heat 

(for the ratio for Venus : Earth will be ^~~ : ^rp that 

is, practically 2:1). We know also that all forms of radiant 
energy are electromagnetic manifestations (see p. 290) and travel 
with the same speed: cosmic rays, y-rays, X-rays, ultra-violet 
rays, light-rays, heat-rays (infra-red), and radio-waves (' ' wire- 
less ") all travel at the same speed, 186,000 miles a second. This is 
one of the most fundamental constants of Nature; the constant 
C (velocity of light) comes into endless mathematical formula 
expressing the laws of science. We also know that unless the 
rays strike material atoms somewhere they remain intact, 
go on indefinitely and apparently for ever. So far so good. 
But what is the nature of radiant energy, how does it travel 
across space at this prodigious speed, and what becomes of it ? 
These are questions which are more easily put than answered. 
Since the days of Huygens and Young (p. 114) the only satis- 
factory solution seemed to lie in the postulation of a "lumini- 
ferous ether/' filling all space including that occupied by matter, 
this ether transmitting radiation by wave-motion; and it may 
be said that the whole of the nineteenth century occupied 
itself with the elaboration of this conception of light- and heat- 
transmission, in terms of waves of different lengths in an 
imaginary ether. The analogy with material wave transmission, 
as in water or as with sound, seemed so^ complete and the 
enormous success of the wave-theory so patent in explaining 
the multitudinous observations of physical science, that little 


doubt was entertained of its fundamental truth. And later, 
since the end of the nineteenth century when shorter waves 
were discovered than those of light and heat, like X-rays, and 
longer ones like radio-waves, these discoveries fitted in so 
perfectly with the theory, by merely extending the length of 
the spectrum of radiation, that the undulatory theory seemed 
complete in its triumph. Just as with sound a whole range of 
octaves of waves are audible to the ear, conveyed by material vi- 
brations, of definite oscillation-frequency for eachnote, so a much 
greater range of octaves of ethereal waves are detectable by 
human sense or scientific instruments, conveyed by non-material 
vibrations of definite oscillation-frequency for each effect. 
All the colours of light, for example, ranging from violet (higher 
frequency of vibration) to red (half the frequency), are thus 
comprised within one single octave out of the hundred or so 
octaves of ethereal vibrations thus believed to exist. Of these 
octaves n of very long wave-length (running into hundreds 
of yards) are used in " wireless " transmission (radio-waves) ; 
17 octaves (from a fraction of an inch to about 14 yards) com- 
prise the so-called Hertzian waves (p. 286) ; 9 octaves (from about 
i to 200 ^*) comprise the heat radiation of the "infra-red" 
region of the spectrum ; and one octave (ranging from 8,000 to 
4,000 Angstrom units*) comprises the full spectrum of light, 
where 4,000 is the wave-length of violet and 8,000 red. On 
the other side of the scale, with still shorter wave-lengths we 
come to about 5 octaves of ultra-violet light (not visible to the 
eye) ranging from 4,000 to 136 Angstrom units; beyond these 
about 9 octaves of X-rays and <y-rays of excessively short 
wave-length, down to *o6 Angstrom unit, and far away beyond 
these come the so-called " cosmic rays" of wave-length in the 
neighbourhood of *ooo2 Angstrom unit. 

Oscillation-Frequency, A very important relation must 
be pointed out here namely, that between wave-length and 
oscillation-frequency. This is best explained by considering 
the analogous case of sound. When middle C of the piano is 

* An Angstrom unit is a convenient one, largely used by science for small 
magnitudes. It is one hundred millionth of a centimetre (which itself is 
rather less than half an inch) -i.e., one A.U. - io- 8 cm. One /* = " 


struck, if the piano is at concert pitch, the sound emitted is 
found to correspond to 256 complete vibrations per second; 
when C-octave is struck in the treble the vibrations are exactly 
double this viz., 512 per second; and when C-octave in the 
bass is struck the vibrations are exactly half of middle C 
viz., 128. That is to say, within any octave the vibration- 
frequencies may have all values between a given number and 
just double; and this applies equally to the range of frequencies 
of all the many octaves of ethereal vibration. From the longest 
waves to the shortest there is a continual increase in frequency, 
which doubles itself for each octave as the scale is descended. 
Thus the shorter the wave-length the higher the frequency. 
To understand this more clearly let us return to sound. The 
velocity of sound is independent of the wave-length or frequency 
it is about 1,100 feet a second for ordinary air conditions, 
but varies with temperature and density. A moment's thought 
will make it clear that if a short wave of sound and a long one 
have got to travel at the same pace by oscillatory wave trans- 
mission, the short wave must oscillate more rapidly than the 
long wave to keep up. It is like a short-legged and long-legged 
pair of people walking together. If they are to keep the same 
pace either the short man must take quicker steps or the tall 
man slower steps, if each keeps to his natural stride-length. 
To put the matter in simple mathematical terms, the oscillation- 
frequency is inversely proportional to the wave-length, and if 
the velocity is known we can say for all cases : 


where F is the frequency of vibration, V is the velocity, and W 
is the wave-length, whose reciprocal is the wave-number. Apply- 
ing this to so-called ethereal vibrations it is easy to see how, 
since V and W are known with accuracy for all points of the scale, 
the frequencies can always be calculated. It will also be easily 
seen that the frequency is quite small for B.B.C. radio-waves 
namely, of the order of a few hundred thousand per second; 
larger (hundreds of millions to a million million) for short 
Hertzian waves; about 100 million million for heat-waves; 
still greater for light waves; of the order i to 30 million million 


million for X-rays and ^-rays; and inconceivably rapid for 
cosmic rays. The figures for these oscillation-frequencies 
(i.e., beats to the second) convey nothing to the human mind; 
they are too big, but they are of transcendent importance in 
mathematical analysis. And whether the undulatory theory, 
which involves these figures, be true or whether, as later 
discoveries seem to indicate, it is merely a convenient device, 
Science cannot dispense with waves and oscillation-frequencies. 
The present position of physics is in a sense a contradictory one, 
inasmuch as it requires the undulatory theory on the one hand, 
as well as a modification of Newton's corpuscular theory on the 
other, to explain the facts. The theory, known as the quantum 
theory (see p. 264), discards the ether as unnecessary, and it 
is just this contradiction which makes it impossible to say as 
yet what radiant energy really is or how it travels. The 
quantum theory, first developed by Planck, is necessary simply 
because the new facts of the twentieth century could not be 
adequately explained by the undulatory theory. 

In order to follow the quantum theory and its more recent 
developments it is necessary to refer first to matter as a radiant 
source of energy and to the dimensions of material particles. 

Matter, as we have seen, may be analysed into ultimate 
units. Masses within the range of our eyesight may, with the 
aid of the best microscopes, be seen to be composed of particles 
as small as 0*2 ^ in diameter ([i=TsW nun.); these particles 
are in turn composed of molecules whose diameters lie some- 
where between 0*1 and 0*5 pp (^=nnrornnr m iL); molecules, 
again, are composed of atoms which themselves are con- 
structed out of vastly smaller protons and electrons, and there 
our analysis stops. And radiation has been traced to the 
oscillatory movements of these protons, electrons, atoms, or 
molecules, as we shall see. 

Heat Radiation. Even when matter is only warm it is 
in a condition to radiate. The surface of any material object 
is radiating when it is what we call cold, even excessively cold, 
so long as the temperature is above absolute zero (-273 C.) 
i.e., it is losing radiant energy and therefore getting colder, 
provided that it is not receiving heat, say, from some other 


radiating surface. The hot atoms or molecules vibrate as we 
have seen (pp. 229, 230), and the point to bear in mind is that 
these vibrations set up oscillatory waves, which pass out as 
radiation (heat) into space i.e., energy, which is lost. It is 
something like a vibrating tuning-fork sending out soundwaves 
into the air (a rather crude parallel) ; and so any material object, 
in the absence of other material radiating obj ects near or distant, 
will lose any heat it possesses by radiating its energy into 
space. It would, in fact, ultimately cool down to a temperature 
of absolute zero, where there is no longer any heat energy. 
This radiation is from the surface, but as heat is conducted 
from within to the surface the whole material object cools down, 
as a red-hot poker cools. The latter does not cool down to 
absolute zero, because its surroundings supply it with heat long 
before this, partly by radiation and partly by conduction and 
convection. Its final temperature, if the poker were in a 
vacuum so as to exclude the complication of conduction, would 
be that temperature of equilibrium at which it radiated out 
exactly the same amount of heat energy as it receives by radia- 
tion from its surroundings. 

Now if we examine this heat radiation emitted by any familiar 
cold (or warm) object, or say by a red-hot poker, we find that 
it does not consist of a single wave-length i.e., definite oscilla- 
tion-frequency but of a whole range of frequencies, and if 
we further examine the energy of radiation over this long range 
we find that it shows a decided maximum at a certain position 
in the range (spectrum), this position being determined solely 
by the temperature of the radiating object. 

For example, imagine a sheet of ice on a frozen pond radiating 
energy to a clear night sky. The temperature is o C., and so 
long as there is water beneath, it will remain at o C. The 
maximum energy of the range for this temperature is found to 
have a wave-length of about n /*. For a red-hot poker the 
maximum energy point, in the range of radiation, is found to 
be at about 6 /*, and for an intensely (white) hot furnace, about 
3 ^ This important relation between maximum energy and 
temperature of the radiating object is known as Wien's dis- 
placement law, which states that the wave-length of the radia- 


tion at the point of maximum energy is inversely proportional 
to the absolute temperature. To take a well-known illustration 
of this law, consider the case of the sun. The sun's radiation 
covers an enormous range, but only about 5 octaves penetrate 
through the earth's atmosphere namely, nearly an octave of 
invisible ultra-violet light, i octave of visible light, and about 
4 octaves of infra-red, the so-called invisible heat-rays. All 
these octaves of radiation on absorption by matter on the earth 
give heat among other forms of energy, so that it is quite a 
mistake to suppose that the heat-rays are confined to the infra- 
red; they are merely the invisible heat rays which give up 
their energy mainly as heat. But in this long range of solar 
radiation much the strongest or most energetic point is found to 
be at a wave-length of -5 p, (blue light), from which by Wien's 
law it can be calculated that the sun's surface temperature must 
be about 6,000 C. In the same way it is possible to calculate 
the surface temperature of the stars. 

Another important point to bear in mind is that not only 
does the wave-length of maximum energy, from any radiating 
object, diminish as the temperature rises (i.e., the oscillation- 
frequency increases), but the amount of maximum energy 
increases steeply as the temperature rises; and the total amount 
of energy of all the radiation also increases rapidly with increase 
of temperature. That is why cool objects do not appear to be 
radiating at all and why a brightly burning coal fire gives out 
so much more radiant heat than one barely red-hot. In fact, 
Stefan showed that the total energy of emission is proportional 
to the fourth power of the absolute temperature (T 4 ). 

Strictly speaking, the above observations on the relation 
between quality or quantity of radiation and temperature 
are true only of perfect radiators the so-called black body 
radiation like carbon black. The actual radiation from the 
atoms and molecules of matter really consists of a vast number 
of spectral lines and bands jumbled together, so as to appear 
practically as a " continuous spectrum/' It is just as if with 
a piano some clumsy player jumbled more or less all the notes 
together instead of playing a melody by striking pure notes 
or harmonies. It is just this melody of pure notes which the 


atoms of single elements play (as it were) when stimulated by 
heat, and it is the business of the physicist to disentangle out 
these pure notes and harmonies as spectral lines, when he 
examines experimentally and mathematically the radiation 
emitted by pure substances. In Nature, substances are not 
usually pure atoms but inextricably mixed atoms and molecules, 
and so the radiation emitted by most forms of matter is the 
jumble of lines and bands which constitute a more or less con- 
tinuous spectrum (p- 162). 

While studying the amount of energy in different parts of 
the spectrum of radiant heat Professor Max Planck of Berlin 
University, in 1900, was forced to the conclusion that the energy 
given out must be regarded as made up of separate units or 
" packets/' the size of each packet, or " quantum " as he called 
it, depending on the oscillation frequency of the radiation, in 
the particular part of the spectrum under consideration. Heat 
might, therefore, be radiated off in an exact number of quanta, 
but fractions of a quantum were impossible. The quantum is 
thus equal to a constant value (' ' Planck's constant ' ') multiplied 
by the number of wave-oscillations per second i.e., by the 
frequency. Exactly what the mechanism of the action was 
Planck was unable to say, and it was not until Bohr introduced 
the conception into his theory of the structure of the atom that 
it became possible to visualise what was taking place. 

When it had become quite impossible to reconcile the be- 
haviour of the electrons in regard to the production of waves with 
the " classical " theory, Bohr showed mathematically that if the 
electrons in the atom were assumed to revolve at enormous speed 
in definite orbits, the change in energy by the electron jumping 
from any one orbit to any other fitted in exactly with Planck's 
theory, and also explained why the spectrum of any element had 
definite lines in mathematical sequence. The position of any 
line in the spectrum corresponds to the energy given out by any 
electron falling from an outer to an inner orbit, and the total 
number of lines corresponds to the various combinations of jumps 
which are possible between the several orbits. The orbits 
themselves are spaced at distances from the nucleus following 
a definite sequence (p. 247), and the eccentricity or ellipticity 


follows a definite sequence, all dependent on the degree of 
excitation (temperature) of the atom, as we have seen. 
Moreover, the lines in such a spectrum are themselves com- 
posed of finer lines very close together, and Sommerfeld has 
explained them mathematically by means of Einstein's theory, 
which demands that the mass of a body, in this case an 
electron, must change with varying velocities (p. 295). Not 
only so, but an intelligible conception of the nature of X-rays 
now becomes possible, for these are due to the displacement of 
the electrons in the orbits nearest to the nucleus. 

Absorption of Radiation. When radiation quanta from 
any object meet matter which obstructs them they shake up 
the atoms and molecules of this matter, in such a way that, 
either the atoms and molecules are set in some vibratory or 
rotatory condition, or the electron-components of the atoms 
themselves are disturbed. A rough sort of analogy is a ship 
agitated by the waves of the sea, where there is some sort of 
relation between the length of the waves (distance from crest to 
crest) and the length of the ship. This kind of relation between 
wave-length and size of object involved is a very marked 
feature of the radiation we are considering, the so-called ethereal 
waves. It can be proved mathematically, and it has been 
demonstrated experimentally, that any electrical structure 
like an atom will only be disturbed by radiation, the wave- 
length of which is nearly a thousand times the dimensions of 
the structure (to be precise, 860 times). Only when the wave- 
length is below this limit will the structure be affected and the 
radiation be absorbed as definite quanta of energy, by doing 
the work of agitation. 

Let us take some examples to illustrate this fundamental 
principle. Suppose we take a homogeneous pencil of rays of 
pure wave-length 3 //, (i.e., in the infra-red) analogous to a pure 
single note of sound, and allow these rays to strike a sheet of 
cold water. We find that these rays are entirely absorbed by 
the water, which is warmed in the process ; the electrons of the 
atoms (hydrogen and oxygen) of water are not in the least 
affected, because the diameters of the electron orbits are far 
below -g^r of 3 yu, in dimensions, but the molecules themselves 


are just about these dimensions. These are put into a condi- 
tion of increased oscillation and rotation about their centres 
of gravity, and this manifests itself in a rise of temperature. 

Glass, having larger molecules than water and of different 
shape, is transparent to infra-red and light radiations, so long 
as the wave-lengths are below about 3 p. and the glass is not 
too thick, but it absorbs wave-lengths of 5-6 /*. When the 
sun's rays, which are rich in all these wave-lengths, pass through 
glass those not absorbed will be still capable of warming water 
and other materials able to absorb them. The temperature of 
such objects so warmed, say to 20 C., will emit radiations mainly 
consisting of much greater wave-length (maximum at about 
10 /*), as explained under Wien's law (see p. 262), and, as glass 
is largely opaque to these wave-lengths, these radiations are 
mainly absorbed by the glass. This explains why it is that 
the heat is trapped, as it were, in a greenhouse or garden frame 
it is not merely that the sun heats the interior directly, but that 
this heat cannot freely escape to space by radiation, quite 
apart from any losses of heat by conduction or convection. If 
the glass is coloured, say blue, this means that there are mole- 
cules present (cobalt in this case) which are agitated by, and so 
absorb, certain wave-lengths, like red, of the visible octave 
of light, the energy of these absorbed rays being transformed 
into heat. It should be noted here that ordinary glass has 
molecules of such sizes and shapes that, while transparent to 
white light, it is opaque to ultra-violet light, and as the health- 
bestowing properties of sunlight depend upon all its rays, 
sunlight is to a great extent deficient in these properties if the 
ultra-violet rays are cut out, whether by glass or by a smoky 
atmosphere. The newly invented Vita-glass lets through these 
valuable ultra-violet rays, and it is therefore good for windows 
provided they are kept very clean, but unfortunately after a 
while some Vita-glass gradually loses this special transparency 
to ultra-violet rays. 

Now, imagine the case of the human eye, and let the wave- 
length of the radiation be changed to, say, '8 ^. It is now pure 
(monochromatic) red light, and when it strikes the eye it is found 
to disturb the atomic orbits of electrons of dimensions -^ of 


8 //,, but not those below this limit. It is the disturbance of 
the electron orbits of the atoms of sensitive chemical compounds 
in the retina of the eye that makes ordinary light visible to 
us; and because the orbits of the electrons of silver atoms 
are not disturbed by red light (*8 //,), but are agitated by blue 
light of half this wave-length (and below), the photographic 
plate is similarly affected. 

For similar reasons X-rays (of very short wave-length) 
have a much more devastating effect on the atoms of matter, 
since they go, as it were, deep down into the atom and disturb 
the inner K- and L-rings (p. 251), whilst light only agitates the 
outer electrons. -7- Rays even affect the nucleus of atoms, and 
cosmic rays in some way appear to affect the proton itself^ 
though this is insufficiently understood at present. 

All this applies to the absorption of radiation by the atoms 
and molecules of matter, but the exact converse is no less 
equally true. That is to say, when any atom or molecule is 
agitated it will radiate energy of just that wave-length (or set 
of wave-lengths) which'corresponds to the degree of disturbance. 
If the molecules only are agitated radiant heat (various infra- 
red rays) only will be emitted. This happens when the tem- 
perature rises from -273 C. upwards, but as the temperature 
rises and the electrons begin to be stirred at about 400 'C. shorter 
waves will also appear and the object will show the appearance 
light (incipient red heat). At still higher temperatures the 
electron agitation increases so much as to give most of the rays 
of the octave of visible white light, and the total energy of 
radiation, as we have seen, greatly increases. At the tem- 
perature of the sun's surface (6,000 C.) still shorter waves 
appear as the inner electrons begin to be disturbed. At 
temperatures ranging from 115,000 to 29,000,000 C., such as 
are found within the sun, the innermost K- and L-rings are 
agitated with production of the excessively short wave-length 
X-rays. At the still higher temperatures of the interior of 
dense stars (up to several hundred million degrees C.), the 
nuclei themselves of the atoms are disturbed and 7-rays appear. 
Lastly, cosmic rays, which may arise from the actual annihila- 
tion of protons and electrons, correspond to the unthinkable 


temperature of over 2 million million degrees C., and this 
'hypothesis of annihilation is supported by the fact that the 
: wave lengths of these cosmic rays are about 860 times the 
dimensions of protons. It would seem then that the limit 
of radiant energy is reached when the disturbance actually 
causes the destruction of matter itself (see p. 269). 

Mass and Energy. To make this problem clearer it is 
necessary to explain more fully what a quantum really is. 
Let us take a single packet of energy, the quantum, and 
examine it. It is the amount of energy, measurable but ex- 
tremely minute, which is emitted as radiation when a single 
electron falls from a given outer orbit to an inner orbit (see 
p. 252). Its energy is found to be strictly proportional, as we 
have seen, to the oscillation-frequency and to a constant (h, 
known as Planck's constant) which appears to be one of the 
fundamental constants of Nature. So it is obvious that the 
energy of any single packet or quantum must be relatively 
small for radiation of long wave-length (like infra-red rays), 
much greater for light and ultra-violet light, relatively enormous 
for X-rays and still more so for ^-rays. Now, as will be seen 
later (p. 295) in dealing with Relativity, energy and matter 
are, strictly speaking, interchangeable terms in the last analysis 
of the problem. It is true we are not able to convert matter 
into energy or vice versa to any measurable extent in any 
laboratory process, but it can be demonstrated theoretically 
that such change is not only possible, but that it probably occurs 
on a stupendous scale in the sun and stars. This is tantamount 
to saying that after all matter is not indestructible, and that 
another of the most cherished beliefs of the nineteenth century 
has been shattered. The real truth appears to be that, so far 
as ordinary experience on this earth is concerned, matter 
undergoes no appreciable change of mass in any chemical 
transformations it may suffer, but that the law of conservation 
of mass and energy must be modified to mean that a kind of 
sum of these two is indestructible, and that matter can be 
transformed into energy, in short, annihilated so far as atomic 
constitution is concerned. Conversely we can now definitely 
say that energy has mass, just as matter has, small though 


this mass is. It has been calculated exactly what a definite 
amount of energy must weigh, and vice versa it has been calcu- 
lated what is the amount of energy which would be released 
by the annihilation of I gram of matter (p. 296). The latter 
is something prodigious and would, if it were ever possible to 
utilise it, do the work of thousands of tons of burning coal. 

- Cosmic Rays. To return to cosmic rays, since the wave- 
length of these has been determined and so the oscillation- 
frequency found, the energy of each quantum of these rays can be 
calculated. When this energy is now expressed as its equiva- 
lent in mass the surprising result is obtained of almost exactly 
the weight of one single proton. This, then, would seem to 
confirm the belief (put forward by Sir James Jeans) that these 
cosmic rays arise from the actual annihilation of matter (pro- 
tons) with liberation of the equivalent amount of energy ; and, 
as might be imagined, this energy is at higher potential than 
any other form of energy in the universe. 

These cosmic rays, especially associated with the name of 
Millikan, are entitled to the epithet cosmic because they literally 
pour in upon the earth from the whole universe, and not from 
the sun and stars. They have enormous powers of penetration, 
far exceeding X-rays or even 7-rays, and can pierce through 
many feet of solid lead. According to Jeans, they come from 
the 2 million or so extra-galactic nebulae, which are found 
within the confines of the known universe, and they originate 
in these nebulae by the actual annihilation of matter. Other 
theories ascribe these rays to the creation of matter in cold 
regions of the universe, but there are difficulties in the way of 
accepting these views. As Jeans supposes that annihilation 
of matter is the source of radiant energy of all stars, including 
the sun and nebulae, it remains to be explained why these rays 
do not come from the sun and stars more than from any other 
region of the sky. To understand this it is necessary to refer 
to phenomena recently discovered and known as the Compton 
and the Raman effects. When radiation of any kind, in its journey 
through space whether long or short, strikes an atom or molecule 
several things might happen. The radiation might go on un- 
disturbed, as with transparent substances; it might be reflected, 


with polarisation (p. 117) ; or it might be absorbed, more or less 
giving effects which have already been described. It might 
also be scattered, and it is in connection with scattering that 
the Compton effect has been observed. 

In 1923 Compton showed that when an X-ray collides with 
an electron there is true scattering, and the X-ray behaves as 
if it were a material particle, giving an elastic collision, with 
a measurable recoil of the electron. So striking is this effect, 
that we may say that the rays behave as if they were discrete 
particles, in spite of the fact that, like other forms of radia- 
tion, they show the usual " interference " and " polarisation " 
associated with waves. Indeed, these observations make it 
more difficult than ever to reconcile the manifestly corpuscular 
nature of light with the undulatory theory. It amounts to this, 
then, that the present-day position of physics is paradoxical, 
in the sense that physicists cannot get on without both of these 
apparently opposed theories. 

It has long been known that ordinary light radiation 
exerted pressure upon any blackened object capable of absorb- 
ing the rays, and the familiar toy called the radiometer, in- 
vented by Crookes in 1874, shows this beautifully; within the 
vacuum tube the little vane of four discs, each blackened on 
one side, rotates merrily when the sun shines, because of this 
pressure. Radiation pressure becomes a very important 
thing in the stars, as Eddington has shown; indeed, when the 
temperature is very high, it is so great as to rival the pressure 
(gas pressure or kinetic energy, due to temperature) of the 
atoms themselves; and so it is partly responsible for maintain- 
ing some stars in a distended condition of immense gas balloons 
without collapsing. In the light of the quantum theory, we 
are now better able to understand both radiation pressure and 
scattering. These phenomena force us to believe that the 
quantum is virtually a third fundamental unit of matter, 
which differs from the other two (proton and electron) by 
possessing no charge, but merely an electric field, a mass 
varying with the wave-length (for ordinary light only about 
'000005 that of the electron), and a velocity equal to that of 
ight. This hypothetical unit is sometimes called the photon, 


and the essential fact, upon which its existence is postulated, 
is that when it is scattered by striking electrons, part of the 
scattered quantum has less than its original energy i.e., there 
is a decrease in the oscillation-frequency. 

Scattering in fact differs from reflection by the fact 
that the scattered quanta are only partly the original ones; 
some of the radiation scattered has a higher frequency 
than the original and some (more) a lower frequency, and 
with light radiation the difference between the two latter 
frequencies is found to be the characteristic frequency of the 
substance in its heat (infra-red) radiation. This is the Raman 
effect, and the general result is to " soften " (as physicists say) 
the radiation i.e., to enrich the radiation with lower frequency 
quanta at the expense of higher frequency. A whitewashed 
wall which scatters (as well as reflects and absorbs) sunlight 
may be intolerably hot in summer because of the enriched 
infra-red radiation produced by the scattering. This " soften- 
ing " result is known as the Compton effect, and it will be 
readily understood that if highly penetrating radiation, like 
cosmic-rays and 7-rays, or even X-rays, are generated within 
the interior of the sun or a star, these rays will be so " softened " 
by scattering in their long journey to the surface, by en- 
countering such multitudes of atoms on their way, that the 
final radiation appearing at the surface will be indistinguish- 
able from ordinary temperature radiation (Jeans). 

Very different is the case with the transparent nebulae. 
These are so thinly packed with atoms that their densities are 
lower by thousands of times than the most extreme vacua 
attainable by any laboratory vacuum pump. The nebulae, 
except towards their centres, are in a condition of extreme 
tenuity, and so scattering will be comparatively slight; and 
highly penetrating radiation from within can get out and across 
the universe without much softening. This theory of Jeans is 
of great importance in its relation to cosmogony, and it is 
confirmed not only by mathematical handling of the quantum 
theory but also by observational means, since the intensity of 
cosmic rays appears to be greatest in the direction of the nearer^ 
nebulae such as that in the constellation of Andromeda. 


The original quantum theory of Planck and the Bohr model 
of atomic structure, which promised so well, are not perfectly in 
accord with certain experimental observations of spectral lines, 
and physicists have now begun to doubt whether the electrons 
of an atom can be regarded as real particles, performing similar 
functions in space to those of macroscopic or gross material 
particles. Electrons are altogether too elusive and evasive to 
be susceptible to the same kind of experimental enquiry as, 
say, the planets circling round the sun or even atoms; and 
Heisenberg, who has founded the new quantum theory, would 
go so far as to say that (as Eddington puts it) "a particle may 
have position or it may have velocity, but it cannot in any exact 
sense have both." The nature of the electron at present is 
very much in the melting-pot. 

New Quantum Theory. We saw on p. 248 that the same 
orbit cannot be occupied by 2 electons. It is as if each electron 
instead of being a material point somehow spread itself out so 
as to occupy an entire orbit to itself. With the higher, i.e. outer, 
orbits it is perfectly true that an electron behaves normally 
as a material point of definite mass and of velocity which may 
be determined; but at lowest energy when the electron drops 
into orbits which are relatively near the nucleus this definite- 
ness gradually disappears and indeterminateness appears in 
its place. It now becomes impossible to identify the point or 
actual position of the electron in its orbit. 

This puzzling feature is very disturbing to Science, which in 
the past has prided itself upon the objective accuracy of its 
determinations and freedom from subjective hallucination. 
So a new theory of wave-mechanics is groping its way in the 
attempt to understand the problem, but thus far it has only 
demonstrated that an electron at lowest energy is not like a 
material particle occupying a definite position in space. 

The new quantum theory of Heisenberg (1925), extended 
by Dirac and Schrodinger, involves a recondite mathematical 
system of wave mechanics so abstruse as to be quite beyond 
the scope of this book. In this new system, as postulated by 
Schrodinger, an electron is really a sort of storm centre of waves 
or rippl 63 i* 1 an imaginary sub-ether, such waves travelling 


with speeds (not uniform like ether waves) depending on their 
wave-length (much in the same way as water ripples), and 
modified by mass influence, such as the nucleus of the atom. 
Elaborate calculations based on this new method give correct 
results for all the spectral lines, including those which are in error 
(p. 272) by the Bohr method, and the new theory promises to be 
extraordinarily fruitful. But it takes us to a world of shadows 
so abstract as to outbid even the theory of relativity (seep. 289), 
so that it is impossible to form any mental picture or model 
of what is really happening. Indeed, the further science 
probes into the hidden recesses of the atomic world, the more 
obscure and shadowy does objective reality seem, the less 
material and tangible does Nature appear to be. As Eddington 
says truly: " An addition to knowledge is won at the expense 
of an addition to ignorance. It is hard to empty the well of 
Truth with a leaky bucket." 

Radioactivity. Perhaps the most startling event in physics 
during recent years has been the discovery of the wonderful 
element, radium. In 1895, the French physicist, Henri 
Becquerel, found that a preparation of uranium, whose atom is 
the heaviest known, gave off something that could pass through 
black paper and even thin sheets of metal, and act on photo- 
graphic plates, even when these were kept in the dark. This 
radiation had other remarkable properties; it had the power 
of causing certain substances to give off a phosphorescent glow; 
it generated heat, and had the power of making gases con- 
ductors of electricity. 

Uranium is a hard white metal, which melts at about 
1,600 C. It was discovered in 1789, and isolated in 1842 by, 
Klaproth, professor of chemistry in Berlin, who also discovered 
strontium, zirconium,, tellurium, and other rare metals. 
Uranium occurs in the mineral pitchblende, found in Cornwall, 
Norway and the United States, but especially in Joachimsthal 
in Czechoslovakia. All the compounds of uranium are " radio- 
active/' but the real nature of this extraordinary substance 
was first elucidated by Professor and Madame Curie in 1898, 
when they discovered radium itself. After the accidental death 
of her husband in 1906, Madame Curie, a Pole by birth, 



succeeded him as professor of physics at the Sorbonne in Paris, 
but, after the war, became professor of radiology at Warsaw. 

Madame Curie's first discovery was that another ele- 
ment, called thorium, discovered by Berzelius in 1828, pos- 
sessed radioactive properties like uranium. This element 
figures in the manufacture of Welsbach gas-mantles, although 
the incandescence of the mantle has no connection with radio- 
activity. Madame Curie also found that certain minerals con- 
taining uranium salts were much more radioactive than could 
be accounted for by the amount of uranium present in them. 
One of these minerals was pitchblende, and after prolonged 
labour she managed to extract from it the metallic elements 
bismuth and barium, which, although not themselves radio- 
active, were each associated with a new metallic element. 
One of these similar to bismuth was christened " polonium " by 
Madame Curie, in honour of her native country, Poland; the 
other was " radium/ 1 which was similar in many properties to 
barium. The radium was present in extremely small quanti- 
ties, for It was estimated that not more than an ounce of 
radium could be obtained from 100 to 200 tons of the very 
richest variety of pitchblende i.e., one part in about 7 millions. 
Even before the war, radium bromide cost 15 to 20 per 
milligramme i.e., about T V of a grain. 

The first remarkable fact which appeared was that 
radium gives out energy spontaneously, without appreciably 
altering or wasting away; this emission of energy can neither 
be controlled nor prevented. It has been calculated that a 
gram of metallic radium gives off 100 calories every hour, or 
about 100 thousand million calories during its lifetime ! Here 
we might have one of the sources of the sun's energy, and we 
will later (p. 351) appreciate the vast importance of the radio- 
activity of the rocks when we come to consider modern geology. 
Since the days of Helmholtz and Kelvin science had regarded 
the law of conservation of energy (p. 127) as fundamental, for, 
as Professor Soddy says: "Energy can only be used once, 
nothing goes by itself "; but radium, about 1900, appeared as 
something exceptional. 

Some idea of the nature of this " miraculum naturae " may 


be obtained by examining a very interesting little instrument 
invented by Sir William Crookes, and called by him a " spin- 
thariscope/' from the Greek word meaning a spark or star 
(Fig. 94). It consists of a small brass cylinder, B, having a 
film of zinc sulphide painted on the bottom, Z. This substance 
becomes luminous or phosphorescent when rays from a radio- 
active source impinge on it. A short distance above the zinc 
sulphide there is an adjustable needle, N, the tip of which has 
been merely rubbed against a tube that had 
contained a minute quantity of radium bro- 
mide so that the amount of radium on the 
needle must be, in the true sense of the word, 
quite infinitesimal. Fitting into the outer 
cylinder is an inner one carrying a lens, L. 
If the zinc sulphide film be examined through 
the lens in a dark room, it will be seen 
to glow, and when the eye becomes accus- 
tomed to the conditions the glow resolves ^ 94> _ SPINTHARI . 
itself into innumerable tiny sparks " like SCOPE. 

a shower of meteors on a winter night." 
These sparks are due to the battering of the so-called oc-rays 
(Alpha rays) given off by the radium, each spark caused by the 
disruption of a particle of zinc sulphide when struck by an oc-ray. 
After some months the glow becomes feebler, but this diminu- 
tion is not owing to any loss of power on the part of the radium, 
but literally to the wearing away of the zinc sulphide by the 
perpetual bombardment to which it is subjected. If the film 
be renewed the meteoric display becomes as brilliant as ever. 
The radium may be said never to wear out -at least, the particle 
which the needle carries is estimated to last 2,000 years ! 

The question at once arises, What is it that is shot off, and 
what happens to the radium atoms so disintegrating ? 

Much information relating to radioactive problems has 
been acquired by employing the knowledge we have gained by 
the study of ionisation of gases. If the ends of an insulated 
glass tube containing a gas be connected with the terminals of 
a battery, the electric current does not pass through the gas; 
but if the tube is exposed to oc-rays, the current at once begins 


to flow. The reason is that the a-rays have the power, so to 
speak, of knocking off electrons from the atoms of the gas, 
resulting in the formation of particles of two kinds: (i) nega- 
tive charges or electrons on the one hand, and (2) the remainder 
of the atom or molecule, positively charged, on the other. These 
particles are of course " ions " (see p. 254), and the gas is 
said to be " ionised/' The degree of ionisation can be measured 
by the instrument called an electroscope, such as that devised by 
Professor C. T. R. Wilson of Cambridge 
(Fig. 95). The substance under examination 
is placed on the platform, A. Above it is a 
metal plate, B, continuous with a rod, C, held 
in a vertical position by the bar, H, which 
rests on the insulators, I, I, Attached to 
the top of C is a strip of gold leaf, G, which 
lies vertically but stands out (as shown) by 
repulsion when charged. At the top of the 
enclosing box is a wire, D, which may be 
depressed through the insulating block, F, 
by a spring knob, E, connected with a source 
of electricity. When E is depressed and 
D momentarily touches the top of C, the 
gold leaf becomes charged and stands out, and if the material 
on A be radioactive i.e., capable of inducing ionisation of the 
gas between A and B the gold leaf falls back at a rate which 
may be measured against a scale which has been previously 
calibrated. The rate of fall of the gold leaf indicates the rate 
of leakage of the charge, and this gives the degree of conduc- 
tivity of the gas or its ionisation. 

By placing films of various materials over the radioactive 
substance the penetrating power of the radiations may be 
determined. In this way it has been found that radioactive 
elements may give off " rays " of three kinds called a, p and y. 
The a-rays are easily stopped by a sheet of paper, but p- and y- 
(Beta and Gamma) rays pass through. A thin sheet of alu- 
minium will, however, shut off the p-rays, but it requires a 
layer of metal several centimetres thick to block the y-rays. 
The a-rays are relatively massive, being in fact positively 

FIG. 95- 



charged particles, which have been shown to be atoms of 
helium that have lost their two orbital electrons. They move 
at a prodigious speed, estimated at 10,000 miles a second. 
They thus possess enormous energy, and it is to them that 
the flashes given off by the zinc sulphide in the spinthariscope 
are attributed. It is almost inconceivable that it should be 
possible to photograph the pathway of an object moving at 
such a speed, but it has been done by Professor C. T. R. Wilson 
in the following ingenious way. 

A cannon-ball fired through a field of 
wheat leaves a track behind it which is 
clearly recognisable, although the ball itself 
cannot be seen during its flight. Similarly 
an a-particle, moving several thousand times 
as fast, may be made to leave behind it a 
track revealing the path it has followed. We 
know only too well that fog is due to the 
condensation of moisture on particles of 
dust, soot, etc., in the air, and Wilson took 
advantage of this knowledge in devising his FIG. 96. PHOTO- 
apparatus, the general principle of which is ^ a " RAYS " 
indicated in Fig. 96. The box A with a 
glass roof, G, has fixed on one side, at R, a minute speck 
of radioactive matter, which shoots its rays against the 
opposite wall. The floor of the box is formed by a piston, P, 
which can be lowered into the position P'. The air in the box 
is saturated with moisture, and when the piston is suddenly 
pulled down the air expands and is thus cooled, and cannot 
carry so much moisture. The excess is deposited as a miniature 
fog on the gaseous ions produced by the passage of the oc-par- 
ticles, and the path of the ray is thus defined by the chain 
of microscopic drops' formed on the ions of the gas acting as 

This experimental technique, devised by Wilson, has 
proved to be extraordinarily fertile in the elucidation of the 
once mysterious phenomena of radioactivity. It has enabled 
the tracks and velocities of <x- and p-particles to be photo- 
graphed and determined, not only from radium, but from 


numerous other radioactive elements; indeed, the method lies 
at the root of Rutherford's classic investigations. 

The p-rays are light negatively charged particles which have 
been shown to consist of electrons, travelling with a speed com- 
parable with that of light, while the y-rays (Gamma) consist of 
true radiation (ethereal) like heat, light, or X-rays (travelling at 
the same speed) but of wave-length (p. 259) shorter even than 
X-rays, and consequently they have by far the greatest pene- 
trating power. It will be seen, therefore, that only the y-rays 
are rays in the ordinary sense of the- word, since the a-rays and 
p-rays are truly material particles (ionised helium and electrons 
respectively) shot out with high speed. 

The intensive study of radium and other radioactive 
elements, during the last thirty years, has thrown a flood of 
light upon the inner constitution of the atoms of matter; in- 
deed it is principally owing to the researches of Rutherford on 
the spontaneous disintegration of radioactive elements that we 
have been able to penetrate so deeply into this problem. 
This disintegration is, so far as we know, confined practically 
to elements of high atomic number (84 to 92) with the exception 
of potassium, which is feebly radioactive; but it is thinkable 
that all the elements above hydrogen may disintegrate given a 
long enough time, and that such radioactivity is too feeble to 
be detected. It is significant that the weightier atoms are more 
unstable than the lighter ones, but it must be observed that in- 
stability is by no means a matter merely of the mass of the 
nucleus. Among the radioactive elements some with smaller 
mass have a shorter life than others with a larger mass. There 
must be something in the inner constitution of the nucleus itself 
that contributes to instability, for the life of some of these ele- 
ments is so short as to be only a fraction of a second, whilst others 
of not very different mass have lives of the order of thousands 
of millions of years. Radium itself stands about midway between 
these extremes, and its life may be judged from the fact that 
about half of it changes into something else within 1,690 years. 

Transmutation. And what becomes of these unstable 
3lements when they disintegrate ? Many of them lose a charged 
itom of helium (a-particle), some lose only an electron (p- 


particle), but in either case by so doing each changes into another 
unstable element which in its turn may lose helium or an electron, 
and so on, step by step, till the process stops at what we call a 
stable element, like lead. It will be, therefore, readily under- 
stood that, since the lives of the intermediate unstable elements 
are variable, the result got from any pure radioactive element 
at the start will be, after a time, a mixture of several other ele- 
ments in addition to the original one, all in steady process of 
decay according to their respective lives, whether long or short. 
For example, if a piece of pure metallic radium were put in a 
closed empty vacuum tube, two gases slowly accumulate in 
the tube : one is helium, the atoms of which are ejected as high- 
speed a-particles (from the nucleus of some of the radium 
atoms) ; the other is the element niton or radon, which is the 
residue of the disintegrated radium atoms. This is proved by 
the fa$>t that the atomic weight of niton is 222 i.e., just 4 units 
less than that of its parent radium (226), these 4 units repre- 
senting the mass of the helium atom ejected. The latter is 
flung out as the simple nucleus of its atom i.e., without the 
2 electrons of the K-ring and so it is positively charged ; but it 
soon picks up stray electrons which are always available (p. 276) ' 
when it strikes other atoms of matter, and so it soon becomes 
ordinary helium, which can be identified by its properties. On 
the other hand, the residue, niton, of the disrupted atom of 
radium, must contain 2 electrons less than the latter, for it 
must be remembered that the nucleus of the helium atom has 
2 embedded electrons in it (see p. 248). This deficiency of 2 
electrons brings down the total orbital electrons by 2 and so 
reduces the atomic number by 2. In fact, niton has an atomic 
number of 86, whereas radium is 88. It must have a completed 
outer ring of electrons because it is one of the inert gases like 
helium itself, though a very different kind of gas. 

Niton was at first called radium-emanation, because the 
investigators, Rutherford, Ramsay, and others, could not 
understand it. It was liquefied at a very low temperature, 
giving out a brilliant steel-blue light, and for that reason was 
named by Ramsay niton or "the shining one/' Its true 
nature only gradually appeared, and then it transpired that this 


remarkable gas was a new element of very short life (half -life 
period only about 4 days), going through a sequence of mostly 
very rapid successive transformations into a series of unstable 
elements which were metals. The sequence and periods of 
half-decay (complete change always takes a theoretically 
infinite time) are given below : 

Niton/ j-Radium-A >Radium-B ^Radium-C ^Radium-C^ (also 

(Radon) Radium-C 2 ) 

(a few days) (a few minutes) (less than $ hour) (about 20 minutes) (infinitesimal time); 

Radium- D ->Radium-E -KRadium-F -^-Metallic lead 

(about 20 years) (about a w eek) (several months) 

It will be easily seen that with this sequence of changes, 
mostly rapid, the contents of our tube, originally containing 
pure radium, must gradually change to a strange mixture of 
new unstable elements. The disentangling of the separate 
processes in the sequence, and the discovery of the life periods 
of each, must surely be one^of the greatest masterpieces of 
Experimental skill in the history of Science; but what an anti- 
'climax in the drama revealed ! The alchemists of old tried in 
vain to bring about the transmutation of elements. They 
struggled to obtain gold from lead. And here we find Nature 
herself engaged in the work of transmutation, but in a direction 
opposite to that fondly hoped for by the alchemists. Lead ! 
the common base and despised metal, as end product of 
Nature's alchemy, and from an element (radium) vastly more 
costly than our precious metal gold. 

It seems like irony, yet it is true, and man's insignificance in 
the process is demonstrated by the fact that nothing he can do 
can, in the least, alter Nature's radioactive transformations. 
Neither heat nor any physical treatment at his disposal will 
alter either the speed or the nature of the processes spontane- 
ously taking place. In fact, it can be shown mathematically 
that Man would need to have at his disposal temperatures 
of the order of 100,000 million degrees Centigrade, to produce 
quanta of the requisite high frequency to disturb the nucleus 
of radioactive or any other atoms. 

Returning to the radium sequence of disintegrations, it 
is to be noted that some of these are accompanied by the 


ejection of a helium nucleus (a-particle) as a by-product, just 
as when radium changes to niton. Whenever this occurs there 
is a loss in atomic weight of 4 and a drop in atomic number by 2, 
as already explained for the case of niton. This happens, for 
example, when the metal radium-F, which is Madame Curie's 
polonium, changes to common lead. But some of the changes 
do not involve the ejection of helium; instead, an electron is 
shot out of the nucleus with terrific speed (varying with different 
cases). This happens, for example, with radium-E, which in- 
cidentally is an isotope (see p. 256) of bismuth, when it changes 
to radium-F (polonium). Here there is no change of mass 
(for the mass of an electron lost is negligible by comparison), 
and so these two elements have the same atomic weight, though 
they are quite different in properties (such elements are called 
isobares) ; and it is necessary to explain why they are different. 
Now the nucleus of radium-E is composed of 210 protons and 
127 " cementing " electrons and therefore has a net positive 
charge of 83 (210-127), thus requiring 83 orbital electrons to 
make the neutral atom . When this nucleus loses one electron, the 
newly formed nucleus contains still 210 protons, but of course 
only 126 " cementing '* electrons. The net positive charge of the 
nucleus is therefore 84 (210-126), and as in the neutral atom 
this must be neutralised by 84 orbital electrons, it follows that 
the new atom (radium-F ) has atomic number 84, whilst its 
parent (radium-E) had atomic number 83. The extra electron 
required for the new atom is provided by its surroundings, 
where mobile electrons in a free condition are always available. 
If not so provided the atom would not be neutralised; it would 
lack a valency-electron or be as we say " ionised " (see p. 254), 
forming what we call a positive ion. The essential point to 
bear in mind is that the newly formed element (radium-F) is 
absolutely different from its parent, and has totally different 
chemical properties, just as, say, bismuth (atomic number 83) 
is different from lead (atomic number 82). 

In general, Rutherford's view of the structure of atomic nuclei 
is that they consist of two parts at least, (i) a tightly bound 
inner nucleus of protons and (2) an outside looser system of 
protons. Some of the protons are bound to individual elect- 


rons, as so-called " neutrons " where each proton is somehow 
tied up with a single electron. It is the inner structure (i) 
which gives the minimum packing possible, and which pre- 
dominates in the smaller atoms, whilst the outer system 
becomes more complex the heavier the atom. 

A mental picture of what might be the case is given by the 
suggestion of Aston in 1920, that the inner nucleus (i) consists 
only of neutrons, whilst the outside portion (2) consists of un~ 
neutralised protons. If this is the case, the latter, which 
probably revolve round the inner nucleus, must be exactly 
equal in number to the total unbound electrons of the electronic 
rings, that is equal to the atomic number of the element. In 
any case, as we have seen, the total electrons of the entire 
neutral atom, including those in the nucleus, is exactly equal 
to the total number of protons i.e., the atomic mass. 

But there is a great deal more yet to be found out about the 
constitution of atoms. A very curious thing, as yet unex- 
plained, is the different nature of elements with even atomic 
number from those with odd. Even-numbered elements are 
usually more abundant on earth (oxygen, silicon, iron, etc.), 
and they may appear in several isotopic forms differing in 
atomic weights, which are both odd and even, by several units 
(mercury, for example, has 7 isotopes, varying between 196 
and 204 in mass). Odd-numbered elements appear generally 
in only 2 isotopes (if any), whose atomic weights are generally 
odd and differ by only 2 units. These things are mysteries 
at present, which later research may clear up. 

It is a very remarkable thing that, in the radioactive dis- 
integration of atoms, it is always helium (as a-particle) which 
is ejected from the nucleus and never hydrogen (as proton). 
This strongly suggests that helium nuclei, ready-formed, as it 
were, are present in the complex nuclei of these heavy atoms. 

In 1916, Sir E. Rutherford made use of a-particles to 
explore the nature of the atomic nuclei, and one of his most 
interesting experiments was carried out on nitrogen. If we 
imagine a large number of peas or any other small objects 
moving freely in the air and widely spaced, the chances of 
the most expert marksman, if even he could see his target, 


hitting one of these peas with a rifle bullet would be exceedingly 
small; but if, instead, the gun were capable of discharging 
several hundred small shot at the invisible target, a collision 
between one of these pellets and a wandering pea might take 
place. Similarly, Rutherford exposed the wandering molecules 
of nitrogen gas to the bombardment of the infinitesimally small 
but incalculably numerous a-particles, with the result that some 
collisions between them and the nitrogen atoms in the molecules 
did take place. The terrific blow inflicted by the oc-particle on 
the nitrogen atom resulted in the smashing of the latter and the 
production of two simpler bodies from it. One of these is a 
hydrogen atom, carrying a positive charge i.e., a proton or 
hydrogen atom that has lost its orbital electron, and the other is 
doubtful but appears to be an isotope of oxygen. 

Now this is a very remarkable, indeed the first, example 
of transmutation of the elements by man. It is modern 
alchemy, but its theoretical significance is profound. For 
if oxygen is formed from nitrogen in this way, it must be an 
isotope of common oxygen (O=i6) with an atomic weight of 
17. Each new atom must have been derived from a nitrogen 
atom weighing 14, through a helium nucleus, weighing 4, 
actually penetrating the N-nucleus, staying there, and dis- 
lodging a proton weighing I. Quite recently another isotope 
of oxygen, weighing 18, has been discovered in minute traces in 
ordinary oxygen, so that this element is no longer " pure/' as 
it 'was supposed to be. 

The deduction, from these and similar observations made 
on other elements, is that as already said the nucleus of an atom 
has a composite structure, containing hydrogen nuclei and prob- 
ably also helium-like nuclei in some cases. Rutherford by his 
transmutation experiments has thus brought us back to Prout's 
hypothesis of over a century ago, and all the elements are in 
the long run multiples of hydrogen. 

Applications of Electricity. Although it is by no means 
the purpose of this book to describe the innumerable appli- 
cations of modern physical theories and discoveries to matters 
of everyday life, a brief reference must be made to a very few 
of the inventions that are based directly on these discoveries 



and that are familiar to everyone. Of these, perhaps, the 
most notable are the telegraph, the telephone and wireless 
telegraphy and telephony. We shall confine ourselves to princi- 
ples; there are books in plenty in which details of the apparatus 
used in each case are given in full. 

The Telegraph. Starting with the conception of an atom 
as a positively charged nucleus surrounded by one or more 
orbital negative electrons, or units of negative electricity, the 
electric current may be conceived as a simultaneous passage 
or stream of electrons from atom to atom. The more freely 
the electron can so pass, the better conductor the material is. 
"Magnetism is a force acting in the ether at right angles to this 
electronic stream, so that whenever a current passes along a 
wire an ethereal disturbance is set up round it which is termed 
a " magnetic field." What magnetism is in itself we do not 
know, and no satisfactory explanation has as yet been put 


forward of the relation that undoubtedly exists between electric 
currents and magnetic flux. 

We have seen that, early in the nineteenth century, Oersted 
(p. 137) discovered that when an electric current flowed along a 
wire it had the power of deflecting a neighbouring magnetic needle 
to the right or the left, according to the direction in which the 
current was flowing. It is on this discovery that the electric 
telegraph is based, but it did not materialise until 1837, when 
Cook and Wheatstone,in Britain, and Morse, in America, invented 
The general principle may be readily understood from Fig. 97. 

The battery, B, is " earthed " on one side at E, and is in 
continuity with the key, K, on the other. When the key is 
depressed contact is made, and the current passes along the 
wire, W, to the coil, A, at the receiving station, Y, which in turn 


causes the needle, NS, to be deflected. The circuit is completed 
by " earthing " the other end of the wire at E'. A tap on the 
key will cause a momentary jerk of the needle, while a more 
prolonged pressure will induce a longer deflection. If a code 
of " dot and dash " be agreed upon, indicating letters and figures, 
it is obvious that messages may be transmitted from X to Y. 
These messages are recorded by watching the movements of 
the needle, or by making the needle write its message on a 
strip of paper uncoiled from a revolving drum, and by other 
methods we need not describe. How messages may be sent 
on the same wire from Y to X (" duplex " and " quadruplex " 
systems), and for a 'description of the "siphon recorder/' in- 
vented by Lord Kelvin for use in submarine telegraphy, books 
on the subject of telegraphy must be consulted. 

The Telephone. The telegraph transmits signs only; the 
next problem was how to transmit sounds, and the instrument 
used for that purpose is the telephone. 

Sound travels through air at about 1,100 feet per second; 
it travels five times as rapidly through water, and fifteen times 
as fast through a steel wire roughly, three miles per second 
so that a sound produced at one end of a wire in London would 
be heard in Glasgow, 400 miles distant, about two minutes 
later provided there be no dissipation of sound on the way 
due to molecular vibration of the wire. But such dissipation 
cannot be prevented; hence a simple wire connection between 
any two stations is quite useless unless the distance between 
them is very short. The principle of the modern telephone 
was discovered when, in 1876, Graham Bell found a method of 
transmitting sounds by electrical means. 

In Fig. 98, D is a thin 
iron diaphragm which 
vibrates near a listener's 
ear. Behind the dia- 
phragm is placed a mag- 
net, M, and a coil, C. 

.C. W W' C 1 

D 1 

The to and fro move- * nxa TELEPHONE. 

ments of the diaphragm 

induce currents of varying intensity in the coil by electro- 
magnetic induction w]bdch pass along the wire, W to the 


receiver, which acts in the reverse order, but is similarly con- 
structed. The current causes the magnetic attraction to vary 
and, consequently, the pull on the diaphragm, D', the vibrations 
of which reproduce the same air waves created by the voice at 
the other end. In the modern telephone the receiver is still 
constructed on the Bell pattern, but the transmitter now used 
is what is called a carbon microphone. The flaw in the Bell 
transmitter lies in the fact that the currents produced are of 
very low intensity, and consequently the receiver emits very 
feeble sounds. In the carbon microphone a film of mica 
receives the air vibrations caused by the voice; this film is 
connected with a carbon disc separated from a second, corru- 
gated, carbon disc by a layer of carbon granules. The vibra- 
tions of the mica plate are transmitted to the first carbon disc, 
which induces variations in the state of compression of the 
carbon granules between it and the second disc. These changes 
in the compression of the granules are accompanied by altera- 
tions in their electrical resistance, which cause variations in the 
value of the current sent to the receiver at the other end of the 
line, and hence give rise to vibrations of the disc of the 

Hertzian Waves. We have already seen that the ear, and, 
behind it, the brain is able to receive and analyse the vibrations 
in the air, which we call sound, but that the capacity of the ear 
and brain in this respect is limited and determined by the 
frequency of the waves and the sensitiveness of the aural 
apparatus. Similarly the eye and again the brain behind 
it is adapted to receive and analyse those vibrations of the 
ether which we call light, but the capacity of the retina as 
a receiving apparatus is also very limited. A glance at the 
scheme on p. 107 shows how small a portion of the range of 
ethereal vibration represents the visible spectrum. It is only 
that part which includes wave-lengths between 0*000,078 and 
0-000,038 cm. that affects .the eye, and it is obvious that if the 
source of light be several miles away, even these waves may 
never reach the eye at all, unless, as in a lighthouse, the lamp be 
greatly elevated and various optical appliances be used to inten- 
sify the light. But there remains the entire range of waves beyond 


the visible spectrum, beyond both the violet and the red ends. 
That such waves exist we have already seen abundant evidence. 

In 1863, Clerk Maxwell put forward his famous electro- 
magnetic theory, in which he showed, theoretically, that any 
changes in electrical conduction created disturbances in the 
ether which were propagated outwards into space with the 
velocity of light. In 1887, Hertz, a pupil of the celebrated 
Helmholtz, and Professor in Carlsruhe, succeeded in demon- 
strating the existence of these waves in a very simple way. 
He used an apparatus called an " oscillator " (Fig. 99), con- 
sisting of an " exciter," A, and a resonator, B. The latter is 
merely a wire bent in the form of an incomplete circle, the free 
ends tipped with metal balls, half an inch apart. The exciter 
consists of an induction coil, C, the terminals being connected 
with two wires, ball-tipped and soldered to two plates, d, d. 
When the current is turned on a spark jumps across the gap 
in the resonator, which is placed some distance away. Hertz 
showed that the waves that passed from the exciter to the 
resonator travelled at the speed of light, and had all the 
characters possessed of light, save visibility. These waves are 
called " Hertzian waves," after their discoverer. 

The chief difficulty was to find a sufficiently sensitive 
method of detecting the waves, but this problem was solved 
A c in 1890 by Professor Edouard 

Branly, of Paris. He discovered* 
d that when an electric spark was 

|H produced the electro-magnetic 

disturbance, so set up, had the 
power of altering the conductivity 



of iron filings some distance away. Branly's apparatus con- 
sisted of a glass tube (Fig. 100) closed at both ends and con- 
taining two metal plugs, M M, to each of which a wire was 





attached, the circuit being completed by a battery, B, and a 
galvanometer, G. Between the plugs, fine iron filings were 
loosely packed, A. Branly found that the resistance offered 
to the passage of the current by the metal particles was very 
greatly reduced when an electric spark from an induction coil 
was produced in the vicinity. In 1894 Lodge gave the name 
of " coherer " to this device, since the particles were conceived 
as cohering to each other when under the influence of the 
electric waves. The effect may be demonstrated very clearly 
by substituting an electric bell for the galvanometer, the bell 
ringing only when the electro-magnetic waves impinge on the 
coherer. When the hammer of the bell on its recoil is made to tap 
the tube, the coherence of the particles is broken and the current 
stops, but the particles are now in the condition to receive 
another impulse from the Hertzian waves. Numerous types of 

coherer have been invented 
since these early days of 
' ' wireless telegraphy. ' ' One 
of them was that used in the 
Italian navy, in which the 
metal filings were replaced 
by a globule of mercury placed 
between carbon plugs. When 
the waves impinged on the 
tube the mercury cohered to 
the plugs and completed the 
circuit, the coherence being 
broken when the waves 

Wireless Telegraphy. 
Soon after Hertz's premature 
death, in 1894, a young stu- 
i dent of physics in the Uni- 
versity of Bologna, named 
FIG. IOI.-MARCONI'S FIRST APPARATUS. Guglielmo Marconi, began 

experimenting on Hertzian 

waves. He greatly improved Brainy s coherer, and, having 
obtained an introduction to Sir William Preece, the chief 






engineer of the London Post Office, he induced him to take an 
interest in his inventions, and to experiment on them on a large 
scale. Marconi's first apparatus was built somewhat on the 
principle illustrated in Fig. 101. The transmitter consisted of 
a powerful induction coil, 1C, its wires ending in large knobs 
only -%j inch apart, S. The wave-lengths were one or two 
metres, and their frequency something like 250 million per 
second. The aerial was a sheet of wire netting called the 
" capacity area/' hung from a lofty pole, P, connected with 
one knob of the exciter by an insulated wire, the other knob 
being earthed, E, The receiver was constructed on the same 
principle, only that the coherer, CO, took the place of the 
induction coil, and a battery, B, and a recorder, E., were 
introduced into the circuit. 

In 1899 it was found possible to send wireless messages to 
France, and, after many more or less successful experiments, 
in December, 1902, a readable message was flashed between 
Glace Bay in Nova Scotia and Poldhu in Cornwall. Three years 
later Marconi hit on the idea of introducing horizontal " an- 
tennae " or feelers, in place of vertical wire nettings, and this 
is the form so familiar to us nowadays on land and on ships. 
Since 1905 progress has been rapid and continuous, and now 
it is possible to transmit instantaneous messages from Britain 
to Australia, a distance of 12,000 miles. As in the case of 
telegraphy and telephony by wires, so in wireless transmission 
we can touch only on the general principle, and must leave all 
details to be studied in special treatises on the subject. 


About fifty years ago there was born, at Ulm, in Wiirttem- 
berg, a boy called Albert Einstein, who became an official in 
the Patent Office at Berne. After publishing various papers 
on scientific subjects he was appointed professor of physics 
at Zurich, at Prague and, finally, at Berlin, where he succeeded 
the distinguished chemist van't HofL He had meanwhile 
become widely known as the .originator of a new way of 



regarding the universe, expressed in what is commonly called 
the " Theory of Relativity/' 

In such a book as the present it is quite impossible to 
give a succinct account of Einstein's Theory, and it is safe 
to say that no one except an expert mathematician can under- 
stand it, or could give a picture of it that would be intel- 
ligible to anyone unfamiliar with mathematical analysis. 
Einstein himself, when lecturing on the subject a few years 
ago, said: " I can tell you in one sentence what it is about. It 
concerns the connection or relation between electricity and 
gravitation. It is a purely mathematical theory, and therefore 
inexplicable to a layman." As this book is written for what 
Einstein calls "laymen/' it would seem, at first sight, better 
to ignore the subject altogether, but to do so would be to omit 
what physicists, mathematicians and astronomers agree in re- 
garding as one of the most fundamental conceptions, that science 
has to show in its development, since Newton. 

When Huygens brought forward his undulatory theory 
of light he felt himself compelled to assume that all space was 
filled with an invisible, intangible something which he called 
" ether/' No one has ever been able to demonstrate its 
existence, and yet without assuming that it actually does 
exist, many phenomena could not be explained. Clerk Maxwell 
showed that just as ether was necessary for the transmission of 
light waves, so it was required for the conveyance of other electro- 
magnetic waves which all travel at the same speed as light. 
Maxwell naturally concluded that light was electro-magnetic, 
and that the sensation of light was a cerebral interpretation of 
the beating of ethereal waves of a certain length on the retinal 
terminations of the optic nerve. 

Now if ether fills all space and the earth is rushing through 
it at the rate of 66,000 miles an hour in its journey round the 
sun, it ought to be possible to detect some drift of the ether 
past it. In order to test this two American physicists, in 1882, 
carried out an experiment now known by their names the 
" Michelson-Morley Experiment." They had at their command 
a method of measurement which enabled them to detect 
- inch in sixty miles, but into that method we need not go. 


A simple analogy will show us the principle on which they 

If a river is four miles broad, and if an oarsman rows at 
the rate of four miles per hour, it is obvious that he could 
cross the river and return to his starting-point in two hours. 
If, however, he rows up the river for four miles while the stream 
has a velocity against him of two miles per hour, it is clear 
that he will require two hours to reach his destination. On his 
return journey he has this two-mile current to aid him, so that, 
with his own speed of four miles per hour, he can accomplish 
his task in forty minutes. His time, therefore, to travel up 
the river four miles and down four miles will be two hours 
forty minutes. 

Michelson and Morley arranged a mirror, A (Fig. 102), 
parallel with the line of the earth's motion, and another at right 
angles to it, B, both at the same 
distance from a source of light, L. 
The times taken by a ray of light 

in travelling from L to A and back w Direction of Earth motion 
and from L to B and back were "Drift of the ether 

compared. After repeated obser- 
vations no difference between the 

two periods could be detected, Mirror B 

although the time of the journey 
to and from B might have been 


Source of light 

expected to be very slightly _ 

* ., , ^ - ? r , / FIG. 102. MlCHELSON-MORLEY 

greater, if there were any drift in EXPERIMENT. 

the ether, for the same reason 

that the boatman took forty minutes more in one direction than 

the other. The obvious deduction from these experiments was, 

either that there was no drift in the ether, or that the earth 

carried the ether with it. 

Although the problem was afterwards discussed for many 
years by leading scientists, no advance was made towards its 
solution until Fitzgerald, professor of physics in Trinity College, 
Dublin, put forward a startling suggestion which has led to some 
remarkable results. This suggestion was that the measuring 
instrument itself was also being shortened in the direction of 


the movement through the ether, by such an amount as enabled 
Nature, so to speak, to cheat the observers of the results for 
which they were in quest. Fitzgerald evolved a mathematical 
expression involving the ratio of the velocity through the ether 
to the velocity of light, which gave the contraction necessary 
to account for the negative results obtained by Michelson and 

The next step was taken by Lorentz, professor of physics in 
Leyden, who had been working at the same problem, and had 
carried out a considerable amount of mathematical investiga- 
tion on the subject. Similar researches were also made by 
Larmor, an English mathematician, and it was really on the 
foundations laid by these men that Einstein began to build. 
He took a fresh view of the whole problem and published in 
1905 a remarkable paper which presented an entirely new 
interpretation of the subject. 

Perhaps the simplest method of explaining what the matter 
is all about, will be to give a summary of a few of the results 
that follow from the theory. To fix our ideas, let us suppose 
that there are two persons, A standing on a fixed platform, and B 
on a platform which can be run on a smooth straight track at any 
velocity. Each person is provided with various pieces of pre- 
cisely similar apparatus viz., a 12-inch steel rule, a clock with 
a pendulum, and a cube of iron or any other material. Suppose 
B's platform be moving at a steady velocity; but it must be 
remembered that the phenomena about to be explained become 
of sensible magnitude only when the velocity of B's platform 
is comparable with the velocity of light viz., 186,000 miles 
per second. 

i. Assume that the 12-inch steel rules possessed by A and B 
are lying parallel to the line of motion of B's platform. To 
A, B's rule will appear shorter than 12 inches, and to B, A's 
rule will appear to have shortened by the same amount. On 
the other hand, A's rule will appear unchanged to A, and B's 
rule unchanged to B. In other words, each observer thinks 
that the distance between any two points on the other's plat- 
form is less than it appears to the observer on his own platform. 
The observations of each observer are reciprocal i.e., the 


observations of A with regard to B's apparatus are the same 
as those of B in reference to A's apparatus. It was Einstein 
who first pointed out this reciprocal nature of the observations, 
and showed that it was unnecessary to regard one platform as 
stationary. He further asserted that a fixed platform was an 
impossible conception, because there can be no absolutely 
fixed point in space, and that the only thing that has reality 
is the relative movement between the two platforms. Previous 
investigators, including Lorentz and Larmor, had treated 
the subject on the assumption of one platform being 
fixed, and had not envisaged the reciprocal nature of the 

In order that we may obtain a more precise idea of the 
contraction of the 12-inch rule, let us suppose that the velocity 
of B relative to A is represented by v, and that the velocity 
of light is represented by c. Let / be the length of the rule as 
seen by B, then the length, /', of the rule as seen by A will be 

equal to JX V i-(~) For example, suppose v is equal to 

half c, then the length I', as seen by A, will be ZxV*~(i) 2 or 
0-87 /. The 12-inch rule would, therefore, appear to A to be 
10*44 inches long. 

Since c equals 186,000 miles per second, the value of v in 
the above illustration is obviously enormously greater than 
any velocity used in mechanisms of human construction. At 
the ordinary velocities to which we are accustomed, the con- 
traction would be immeasurably small, and that is the reason 
why the phenomenon has never revealed itself to common ob- 
servation. Further, we can see from the expression that as 
the value of v increases nearer and nearer to the value of c, the 

value of the fraction - approaches nearer and nearer to unity, 


/ / A 2 

and consequently the value of the expression V i - (~) a P- 

/ 7 \lz" 

proaches zero until, when v equals c, V i - (2 J =o. If the 

platform is travelling at the velocity of light, the 12-inch rale, 
the platform and everything on it would appear to have on 


length at all in the direction in which it is moving. If values 
greater than c be inserted into the expression, the value of 

\/ i (-) becomes imaginary, for we should be trying to find 

the square root of a negative quantity; so that we are compelled 
to admit that velocities greater than that of light are im- 

2. Let us now consider the behaviour of the two clocks, 
which are assumed to be identical in every way, so that, if 
placed side by side on the same platform, the pendulums 
would beat at precisely the same rate. But when A observes 
B's clock on the moving platform, he finds that the pendulum 
makes fewer beats per minute than his own clock does, and, 
reciprocally, B observes the same fact with regard to A's 
clock; each observer sees no change in his own clock, but thinks 
that the other clock is beating more slowly than his own. 
This part of the theory is a mathematical consequence of the 
phenomenon described in Section i. 

Expressed mathematically, if t represents the time between 
two beats of A's clock, then f i.e., the time between two beats 

of B's clock according to A's observations, is where a stands 

for the expression V i ( - j . Suppose, as in the previous case, 

B is travelling at a velocity, v, which is equal to \c, and that 
A's clock makes sixty beats per minute, then A would imagine 
that B's clock makes 60x^87 or 52*2 beats per minute. If 
the velocity of B's platform increases, the pendulum of his clock 
will appear to A to beat more and more slowly, until, when the 
velocity of the platform equals that of light, the pendulum will 
appear to A to be stationary. 

One point must be carefully noted viz., that the dimen- 
sions of bodies measured at right angles to the direction 
of the relative motion are quite unaffected by that relative 

3. Consider next the case of the two similar iron cubes. 
When there is relative motion between the two platforms, A 
will think that the mass of B's cube is greater than that of his 


own cube, and, conversely, B will think that A's cube has a 
greater mass than his, though each observer will notice no 
change in his own cube. Thus, suppose A were able to act on 
B's cube by means of a suitably applied force, operating at 
right angles to the lines of motion, A would find that a 
greater force would be required to produce a certain accelera- 
tion, in the line of the applied force, than would suffice to 
produce the same acceleration in his own cube, and, re- 
ciprocaEy, for B. 

If m represents the mass of A's cube, then the mass of 
B's cube, or, to be more precise, the "transverse" mass, will 

appear to A to be . Thus if A's cube weighs 10 pounds 

(masses being compared by comparing their weights), and if, 
as before, B is travelling at half the velocity of light, then, 
according to A's reckoning, B's cube, when an attempt is made 
to cause a deviation in it at right angles to its line of motion, 
has a mass equivalent to nj pounds. An actual example of 
this phenomenon is to be found in the passage of electrons from 
the cathode to the anode in a discharge tube. As the voltage 
between the cathode and the anode is raised, the velocity of the 
electrons increases, and the mass also increases according to the 
above mathematical expression, as can be demonstrated by 
causing the electrons to deviate from the straight path by 
external electrostatic and magnetic fields. This experiment 
was carried out by Sir J. J. Thomson years before Einstein 
put forward his theory, and he obtained a mathematical ex- 
pression representing the increase of mass, but was unable to 
give any reason for it. 

4. As the result of further research Einstein obtained an 
expression which gives the total energy of a body in motion, 
which consists, first, of the energy, a, within the body itself, 
existing apart from the motion, and, second, of the energy, 6, 
due to its motion relative to the observer. What we call the 
" mass " of a body is attributable to this internal energy, a. 
(f Mass J> is the manifestation to us of this internal energy, and 
thus the terms " mass " and " energy " are interchangeable. 
The component* 6, is simply the ordinary kinetic energy of a 


moving body (J miF), familiar in general treatises on dynamics. 
The atomic energy contained in i pound of any material 
i.e., in a mass of i pound would be capable, were it possible 
to release it and use it in a suitable manner, of raising 25,000,000 
tons to a height of 100 miles ! 

This conception is far-reaching. For instance, if 56 pounds 
of quicklime are combined with 18 pounds of water, the result 
is, so far as all experience shows, 74 Ibs. of slaked lime (p. 167), 
plus the heat produced during the combining of these two sub- 
stances, and lost. Now, since heat is a form of energy, and 
since energy is equivalent to mass, the dissipation of heat 
represents a certain loss of mass and therefore of weight, and, 
in consequence, the amount of slaked lime produced must be less 
than 74 pounds in weight; but the difference is so infinitesimally 
small as to be entirely beyond the ability of the most delicate 
balance to detect. The heat given out by the sun must likewise 
represent a loss of mass by the sun, and that is the view held 
by modern physicists, and it doubtless accounts largely for its 
immense heat resources (see p. 309). The same thing also 
applies to the stars. 

5. Suppose that B has a small trolley on his own platform, 
carried on a track on this platform, lying parallel with the 
track on which his own platform runs, and suppose that this 
trolley is running on its track in the same direction as that in 
which the platform is running, what velocity will the trolley 
appear to have relative to B and relative to A ? If the velocity 
of the trolley be % relative to some point on its own track, then 
this is the velocity which B would ascribe to it. Let the 
velocity of B's platform relative to A be v^ then it would be 
natural to suppose that the velocity of the trolley relative to A 
would be #1+^2- Taking ordinary velocities, let v be ten 
miles per hour and v% be twenty miles per hour, then from A's 
standpoint the velocity of the trolley is 10+20=30 miles per 
hour. But, according to the theory of relativity, this is not the 
case; and just as length, time and mass are insensibly affected 
at ordinary terrestrial velocities, so they axe in the above 
example for the number chosen. If, however, the velocities 
Vi and 0g are of enormous magnitude i.e., comparable with 


the velocity of light the velocity of the trolley, according to 
A, would be less than Vi+# a * an d would be : 

Let V-L=^C and t; 2 =ic, then v$=%c or o*66c. If the veloci- 
ties were simply added, as in the first example given, the 
velocity of the trolley relative to A would be 075^. According, 
therefore, to the theory of relativity" the velocity of the trolley, 
relative to A, is less than that given by the ordinary method of 
combining velocities. 

In 1851 Fizeau, an eminent French physicist, carried out 
an experiment in which two rays of light were passed through 
moving columns of water contained in long glass tubes. The 
velocity of the flow of the water was the same in each tube, 
but the direction of the flow in one tube was the same as the 
direction of the light ray, and in opposition to it in the other. 
It might naturally be expected that the velocity of the light 
in the first tube would be increased by that of the water, and 
decreased in the second by the same amount. But actually it 
was found that the effect of the velocity of the water was 
entirely different, and although a mathematical equation was 
formulated which agreed with the tests, and which is, incident- 
ally, derivable from that given above for the compounding of 
velocities, the explanation remained a complete mystery until 
Einstein found the solution, wherein he used the new method of 
combining velocities. 

It is interesting to study the effect of substituting various 
values in the equation given above. Suppose, for example, 
that B directs a ray of light from a lamp on his platform in the 
direction of movement of his platform, what will the velocity 
of this light appear to be to A ? Substituting c for ^ in the 
equation, we obtain 



that is to say, the velocity of the light as viewed by A is 
unaffected by the velocity of B's platform. 

Let us next imagine that B's platform is travelling with the 
velocity of light, what will now be the velocity of the ray of light 
projected from B's platform, so far as A is concerned ? This 
means that both Vi and v 2 have now the value of c in the 
equation, and on substitution we obtain 

so that we arrive at the velocity of light once more. 

From this we conclude that the velocity of light (in vacuo) 
is constant and the same for aU observers, whether they 
regard themselves as stationary or in motion. Surprising as 
this result may seem at first, it is only what one would deduce 
from the final interpretation of the Michelson-Morley experi- 
mentviz., that the velocity of light in vacuo is a constant. 

6. In Section i we tried to explain how relative motion 
affected the apparent length of a body, or, in other words, the 
apparent distance between any two points in the line of motion, 
and in Section 2 the apparent period of time between two 
occurrences. Suppose that two observers in motion relative 
to each other observe the same phenomenon, say, on a distant 
planet, how will the phenomenon appear to each observer ? 
Suppose that A regards himself as stationary, and that it is B 
who Is in motion, then the distances measured on the planet 
will appear greater to A than to B, whereas intervals of time 
between occurrences will appear less to A than to B. When the 
observers meet and compare notes their records of the same 
phenomena will not agree. Can their readings be reconciled ? 
They can, but only by using mathematical expressions and 
conceptions which combine both distance and time, or, as 
properly expressed, space and time. This combination gives 
rise to the conception of a space-time continuum i.e., an all- 
pervading extension analogous to ordinary space, but having 
four dimensions, three in space and one in time. 

It is quite impossible to form a mental picture of a space-time 


continuum; it is purely a mathematical entity. In the space- 
time continuum occurrences are called " events/' and the 
separations between them " intervals." In the case suggested 
above, the two observers would now find that the intervals 
between successive events, as witnessed by them separately, 
would agree; and if expressed mathematically in terms of 
space and time, all natural laws would be the same for all 
observers, no matter whether they were on the same or on dif- 
ferent platforms, moving relatively to each other, a condition 
of affairs that cannot be realised by using the expressions of 
classical dynamics (Newtonian space and time). 

In all the results we have given above of the effect of 
relative velocity on the observations of A and B, it was assumed 
that the relative velocity remained the same during the period 
of the observations. This part belongs to what Einstein calls 
the " Restricted or Special Theory of Relativity." Only a 
few of the implications of this part have been mentioned, but 
these, it is hoped, may have given a general idea of the nature 
of the subject. 

Einstein next tackled the much more difficult problems 
coming within the scope of the "General Theory." The 
General Theory, which includes the Restricted Theory, was 
published in 1915, and deals with problems in which there is not 
a constant relative velocity, but, instead, a constant relative 
acceleration that is, a constant change in velocity between 
the observers, and on this work he based his famous theory of 

Gravitation appears to us as a force of attraction between 
the earth and all bodies on its surface. (We need not concern 
ourselves here with the attraction between the heavenly 
bodies.) Newton regarded gravitation as a force, and his 
dynamics are based on the assumption of a force of attraction 
existing between all bodies; but the seat of the force, how it is 
produced and how it acts on these bodies, have always been 
profound mysteries. 

As we explained before, there are forces existing be- 
tween electrically charged bodies, such as pithball and a 
rubbed glass rod, which resemble gravitational force, but with 


at least one great difference viz., they not only attract but 
may also repel. Forces of attraction and repulsion also exist 
between magnets, and in the case of pieces of iron, nickel or 
cobalt in a magnetic field. But, unlike electric or magnetic 
forces, gravity always attracts and never repels. It acts on all 
bodies, no matter of what substance they are made, and gives to 
all of them, when free to fall, exactly the same acceleration. 
These characteristics are of remarkable significance, as we shall 
see later on; meanwhile, let us make a few hypothetical 
experiments in order to clarify our ideas. 

Suppose we have a trolley running on a straight, level track, 
and that when in motion there is no frictional resistance due 
to the axle bearings of the trolley on the track, and no resistance 
from the air. (Of course, in making these suppositions, we 
lay ourselves open to the criticism that the wheels and the axles 
of the trolley ought to be assumed to be massless, but we may 
offer the excuse that we are not writing for experts.) Now we 
know from everyday experience that the trolley will not start 
from rest unless we give it a push or a pull in other words, 
unless we apply force. It is also common experience that it 
requires a greater force to set a heavy trolley in motion than a 
light one. What is the precise effect of applying different 
amounts of force to the trolley the line of the force being, 
of course, that of the track ? The trolley contains a certain 
quantity of matter or material; we think of this as "mass," 
and the force of gravity acting on this mass gives it what 
we call " weight/' In everyday life masses are compared by 
estimating their weights, although this is not the only pos- 
sible way of comparing them. 

Let the weight of the trolley be 322 pounds (this number 
is selected simply because it renders the arithmetic easy). If 
we apply a force, either a push or a pull, of 10 pounds (the 
force being equal to that of this weight) along the track, 
the trolley will start from rest, and, as long as the same force 
is continuously applied, the velocity of the trolley along the 
track will steadily increase at the rate of i foot per second in 
each second. The force is 10 pounds, and the acceleration is 
unity; the ratio between them, therefore, is 10 to I. 


If we alter the value of the force this ratio will still hold 
good, and, hence, we should find that with a force of 322 pounds 
i.e., a force equal to the weight of the trolley the accelera- 
tion would be 32*2 feet per second in every second. 

But this acceleration is what is found by experience to be 
the case when a body falls by the force of gravity (strictly 
speaking, at London). So we see that when the applied force 
is equal to the weight of the body, the acceleration is the same 
as that which would be produced were the body allowed to fall 
in the earth's gravitational field; in other words, we have pro- 
duced artificially what would be done by the natural attraction 
of gravity of the earth. 

Obviously if we double the mass of the trolley we must 
apply twice the force to obtain the same acceleration, and that 
is precisely true of weight in the earth's gravitational field. It 
is not of the slightest consequence of what material the trolley 
is made; all that matters is the mass, and again this agrees with 
our experience of the force of gravity. 

It is important to note that, in the experiments with the 
trolley, the force of gravity acts perpendicularly to the line of 
the applied force and of the motion, and therefore it could have 
had no effect on the results, which would have been the same 
had the experiments been carried out in interstellar space, 
where a gravitational field may be neglected. The trolley 
would have no "weight" in interstellar space, although it 
would still have the same mass as before. 

It is this " mass " that gives to all bodies their property of 
" inertia " i.e., their unwillingness to move, if at rest, unless 
compelled to do so by the application of a force, and also their 
unwillingness to stop, if in motion, unless a counterforce be 
applied, which will produce a " deceleration " or decrease in 
velocity of so many feet per second in every second. The 
counterforce must cease the instant the body comes to rest, 
otherwise it would immediately afterwards begin to accelerate 
the body in the reverse direction. (Note that the terms " in 
motion " and " at rest" are to be taken as relative to the 

Most of us remember Jules Verne's story " From the Earth 


to the Moon/' and how the hero of the tale is shot out of a 
gigantic cannon, pointed vertically, in a projectile as large as 
a small room. Let us imagine ourselves in such an immense 
shell, but that instead of being shot upwards at a prodigious 
velocity, the projectile has an upward acceleration of 32*2 feet 
per second in every second, gravitation in the reverse way, so 
to speak. At the same time let us assume that the force of 
gravity of the earth does not exist. Inside the projectile we 
should experience a force of exactly the same nature as we 
do in the earth's gravitational field. 

For example, a body resting on a table in the projectile would 
be urged upwards, along with every other thing in the pro- 
jectile, with an acceleration of 32*2 feet per second, and would 
press on the table by the same amount as it would do on the 
earth's surface in its gravitational field. If pushed off the 
table the body would have no support to sustain the pressure 
it exerted on the table, therefore no applied force to urge it; 
it would lag behind the other moving bodies, and, to us in the 
projectile, would seem to fall to the floor with an acceleration 
of 32*2 feet per second in every second, exactly as a falling body 
does on the earth's surface. This, then, is the meaning of gravi- 
tation it is the result of an acceleration. Professor Eddington 
tells us that " gravitational fields of force are illusions. The 
apparent force arises solely from acceleration, and there is 
nothing of gravitational force at all. A gravitational field of 
force at any point in space is in every way equivalent to an 
artificial field of force resulting from acceleration, so that no 
experiment can possibly distinguish between them." 

Now, a gravitational field can not only be created, it can 
also be destroyed, for were the projectile to fall back to earth 
under the attraction of gravity it being assumed that there 
was no air resistance and that the fall was absolutely free 
we, in the projectile, should find that all evidence of a force of 
gravity had entirely disappeared. A body released from our 
hand would seem to float in space, because it would be falling 
towards the earth, along with the projectile, at exactly the 
same speed. A body pushed off the table by a force acting 
parallel to the floor would cross the projectile in what we, 


inside the shell, would regard as a horizontal direction and hit 
the opposite wall at the same level as that of the table. To 
an observer stationed on the earth, however, and able to see 
through the wall of the projectile, the body would seem to 
describe a parabolic curve, just as a body thrown horizontally 
at the earth's surface would do. 

Reverting to the rising projectile, it would appear that in 
order to produce the effects we observed therein on the surface 
of the earth, the earth would have to increase in size with a 
constant acceleration that is to say, the radius would have 
to increase at the rate of 32*2 feet per second in every second. 
But we know, of course, that the earth is not behaving in this 
grotesque manner. What, then, is taking place ? Einstein 
holds the view that the phenomenon of gravity, acting on a 
body, is not attributable to the attraction of the earth at all, 
nor to an increase in the length of the radius of the earth, but 
to the condition of the space-time continuum in the neigh- 
bourhood of the body. The idea is analogous to that put 
forward by Clerk Maxwell for electrically charged bodies. 
Maxwell considered that the seat of the phenomenon was out- 
side, and not inside the body. For instance, he thought of the 
energy of an electric current that was being carried by a con- 
ductor, as residing in the field outside the conductor, which 
simply acted as a guide to the current. 

Einstein developed a number of equations which describe 
the properties of the space-time continuum where a gravi- 
tational field does not exist say, for example, in interstellar 
space and also a number of equations which describe the pro- 
perties of the space-time continuum in gravitational fields. In 
the case of the former the mathematical forms of the equations 
resemble those appertaining to geometrical figures drawn on a 
flat surface, where the continuum is, so to speak, " flat "; in 
the second case, the forms resemble the ordinary equation for 
figures drawn on curved surfaces i.e., a sphere and the con- 
tinuum in a gravitational field is " curved/' This explains the 
statement circulated some years ago that Einstein had said that 
space was " curved." Einstein was referring to the space-time 
continuum and not to space in the ordinary sense of the term. 


It is impossible to visualise either a fiat or a curved con- 
tinuum, and the terms " flat " and " curved" are, as we have 
already tried to explain, based simply on the analogy between 
space-time equations and those used in ordinary mechanics. 

Einstein made three predictions by which he said his theory 
might be tested. They were as follows : 

1. The perihelion of the orbit of Mercury moves round the 
sun in the course of centuries. The observed advance is 574 
seconds per century, but Le Verrier (p. 87) calculated that, in 
accordance with Newtonian dynamics, the time should be 
532 seconds ; and since his day the solution of the discrepancy of 
forty-two seconds has baffled the ingenuity of all the mathema- 
ticians and astronomers. Working on his new theory, Einstein 
has repeated these calculations and showed complete agree- 
ment to exist between observation and calculation. 

2. Light is a form of energy; it is of the same family as 
heat, and from what has been said (see pp. 268-270), it follows 
that light has mass. It, therefore, must be subject to gravita- 
tion. Keeping this fact before him and using his new theory 
of gravitation, Einstein predicted that a ray of light coming 
from a distant star and passing close to the sun's disc would 
be displaced by 175 seconds of arc. In order to test this, 
observations were made during the eclipse of the sun which 
took place on May 29, 1919, and as a result of this and 
later eclipses astronomers are satisfied that Einstein's predic- 
tion is fulfilled. , ' W}'? ,;>$' *>'< 

3. A spectral line should be displaced towards the red end 
of the spectrum by an amount, depending on the strength 
of the field of gravity through which the light passed. The 
strength of the gravitational field of the sun is immense, and to 
test the theory observations have been attempted on sunlight. 
The experiment is, however, one of extreme difficulty; and, so 
far, no definite results have been obtained one way or the 
other, doubtless because the sun's gravitational field, great as 
it is, is not sufficiently intense. Such intensity can only be found 
in the case of massive dwarf stars (p. 310), and it is interesting 
to note that recent confirmation of the spectral shift, predicted 
by Einstein, has been found in the case of the massive dwarf 


companion of Sirius, the density of which is prodigious and 
whose gravitational field at the surface is therefore enormous. 

The advent of Einstein's theory implies that many ideas of 
the older dynamics must be scrapped, but its refinements, which 
are necessary in modern physics and in certain astronomical 
work, are negligible in many applications of science. On the 
cither hand, Relativity, by its implications, has rendered obso- 
lete the old system of Philosophy based onNewton andDescartes, 
also many familiar conceptions, as we shall see (p. 338) when 
we come to consider astronomical space and time. 

We have given considerable space to the discussion of this 
very difficult and abstruse subject, but some explanation 
seemed called for in view of the great interest aroused by 
Einstein's theory since it was first promulgated, and the im- 
portance ascribed to it at the present day by astronomers, 
physicists, philosophers, and mathematicians alike. 


The great advances made in astronomy during the latter 
half of the nineteenth century were mainly due to the greater 
instrumental perfection of the telescope and spectroscope, 
aided by photography; and by these means an immense mass 
of information was accumulated concerning the distances and 
nature of stars and nebulae. But it is only within the last 
thirty years, with the aid of newer instruments, such as the 
great loo-inch telescope at Mount Wilson and the interferometer 
of Michelson, used in the clear skies of America, that data have 
been obtained enabling astronomers and mathematicians to 
bring the universe into true perspective. We can, of course, 
only give a bare survey here, dealing with the nearest objects 

The Moon. The first object in the heavens that Galileo 
studied with his newly discovered telescope was our nearest 
celestial neighbour the moon. Astronomers from his day, 
almost without exception, down to quite recent years have 
always regarded our satellite as a small dead world, without 



any atmosphere, and showing no activity of any kind. There 
was no water on it, and hence there could be no denudation; 
the volcanoes were all extinct; and since there was no water 
and no air there could be no life. It was nothing but a rough 
stone swinging round the earth once a month and passively 
reflecting the sun's rays to us when it was in the proper position 
to do so. The only one of any importance who ventured to 
express the possibility of there being life on the moon was the 
great astronomer Sir William Herschel, but his views on the 
subject were only vague and hesitating. 

In 1887 a great work on the moon was published by two 
German astronomers, Madler and Beer, illustrated by a pro- 
fusion of maps giving details of the mountain ranges, volcanoes 
and plains, and these authorities came to the conclusion that 
the lunar world was " changeless, airless and lifeless/' But 
this view was not accepted by some observers, who thought 
they had detected alterations in the form of some of the volcanoes 
and to the 'great plain called " Plato " in the lunar maps. One 
-of these was Professor Pickering of Harvard University, who, 
in 1900, established an observatory in Jamaica. He brought 
to his aid the art of photography, which had grown to be a 
valuable asset to the astronomers of the last decades of the 
nineteenth century. Pickering concluded that the moon was 
not a dead world, but that it had an atmosphere, though an 
extremely rarefied one, containing carbon dioxide and water 
vapour, and that some of its volcanoes were still feebly active, 
giving out clouds of dense gases. The white linings of some of 
the craters he believed were due to hoar frost, frozen water 
vapour that had never gone through the liquid condition, and 
he urged the possibility of the existence of vegetation of a very 
low type. We now know, however, that the moon is too 
light an object to retain any appreciable atmosphere (see p. 361) ; 
and without this, life as we know it is impossible. 

The Sun. One of the first obvious facts known about the 
sun was the existence of spots on it, by watching the movements 
of which it was discovered that it revolved on its own axis once 
in about twenty-five days. By the seventeenth century, also, 
the sun's distance from us was made out to be 87 million miles, 


certainly several millions too little, but not a bad approxima- 
tion. In the eighteenth century Sir William Herschel studied 
the spots more carefully, and concluded that they were 
gigantic holes through which the dark solid core was visible 
(p. 163). After Herschel's time, about the middle of the nine- 
teenth century, a German apothecary, called Schwabe, after 
observations extending over more than forty years, estab- 
lished the fact that the number of sun spots passed through a 
cycle lasting for about eleven years, and interest in these spots 
increased when it was discovered, soon afterwards, that the 
rise and fall in their numbers were coincident with magnetic 
variations on the earth. 

Another curious discovery made about the same time was 
that while the sun was rotating at the equator in twenty-five 
days, the region between the equator and the poles lagged 
behind as much as two and a half days, and considerably more 
nearer the poles; further, that the belt of spots moved gradually 
closer to the equator and then died out, while new belts appeared 
in higher latitudes. 

By this time astronomers had distinguished next the solar 
body an envelope which was called the " photosphere/* or 
light-giving layer, surmounted by a " chromosphere," or solar 
atmosphere proper, composed of less heavy gases, outside 
which again was a silvery haze or " corona," visible only at 
times of total eclipse. From the chromosphere shot out from 
time to time immense red flames (hydrogen), which reached a 
prodigious height. 

After 1860, when Kirchhoff had provided us with that 
invaluable instrument, the spectroscope, Lockyer, it may be 
remembered (p. 164), discovered the new element helium, in 
the chromosphere, which was identified, in 1898, as a con- 
stituent of the earth by Ramsay. Another notable discovery, 
made by Doppler of Prague, also about the middle of the 
century, was that the lines of the spectrum were displaced 
towards the violet end when the source of light was approaching, 
and towards the red end when it was receding, and thus we were 
able to follow to some extent the movements of the so-called 
" fixed stars " in the heavens (see p. 323). 


In 1891 a new instrument was brought into use in the in- 
vestigation of the sun. This was the spectro-heliograph, the 
invention of G. E. Hale, the Director of the great observatory 
at Mount Wilson, California. Professor Sampson of Edin- 
burgh, the Astronomer-Royal for Scotland, describes this 
apparatus in the following terms: "In the photograph of a 
spectrum each line is a record of the presence and the state of 
a separate chemical element at the spot on the disc to which the 
slit is directed. If this record could be read for that special 
line for the whole disc, we should have the same information 
summed up for the whole sun. Let the light from the line in 
question be allowed to pass to the photographic plate, by means 
of a second slit, at the focus of the camera, the jaws of which 
shut off aU the rest of the spectrum. Let both the first and the 
second slits be long enough to extend right across the image of 
the sun. Move the image of the sun across the first slit, then 
the light which passes through the second slit will come at 
every moment from different strips of the sun's surface; and 
if the photographic plate be moved behind the second slit, 
in unison with the movement of the sun's image across the first 
slit, a record will be given, not of the radiations of every sub- 
stance mixed together, as in ordinary photographic or visual 
observations of the sun's disc, but of the states of some isolated 
substances, such as hydrogen or calcium, and even of different 
strata of these." 

With the aid of the magnificent equipment of the Mount 
Wilson Observatory, Hale was able to announce in 1908 " that 
the sunspots were caused by vortices in the solar atmosphere." 
In 1896 Zeeman of Amsterdam had shown that the lines of the 
spectrum are widened and even split into several lines when the 
light comes under the influence of a strong magnetic field. Hale 
applied this ' ' Zeeman effect/' as it was called, to the sunspots, 
and was able to show that the vortices were electrical, and in 
1922 he went a step further by noting that the spots were 
associated in pairs of opposite magnetic polarity. 

So much for the surface of the sun. What is it like inside, 
and how does it compare with other suns i.e., stars ? The 
answer is to be found in the wonderful mathematical analysis 


by men like Emden, Eddington, and above all by Sir James 
Jeans, as revealed in his recent books on the nature of the 
universe (Cosmogony and Astronomy, and The Universe Around 
Us) . The story, so far as the sun is concerned, makes the latter 
to be a very average kind of star, born some 5 or 6 million 
million years ago along with other stars out of a gigantic nebula, 
the sun being then considerably larger and more massive 
(perhaps three or four times heavier) than it now is. During 
this long period of time it has been (and still is) wasting away, 
relatively quickly at first but more slowly now, by the anni- 
hilation of the matter of which it is composed (see p. 269), with 
emission of a corresponding amount of radiation energy. 

There is no other imaginable source of energy which could 
last so long, and Jeans supposes that it is mainly elements of 
atomic number 95 that contribute to this annihilation that 
is to say, elements which do not exist on the earth (uranium, of 
atomic number 92, being the highest). These hypothetical 
massive atoms will mainly be found in the central region, which 
is the principal store of energy, and on this account they do 
not appear in the surface spectrum or on the planets which 
were born out of the surface layers of the sun, as we shall see 
later (p. 332). 

On Jeans' theory the sun is not to be regarded as a ball of 
glowing gas greatly compressed at the centre, as Emden and 
Eddington regard it, but rather as a " liquid " core, of density 
about 140 times that of water, surrounded by a gaseous envelope, 
the outer surface of which we see. But this liquid core has very 
different properties from any liquid we are acquainted with on 
earth. One of the densest liquids we know is mercury (less 
than fourteen times that of water), but in the material of some 
so-called " dwarf stars/' like the small companion of Sirius, 
we have densities of 50,000 or even more. So here is some- 
thing very strange. 

What, then, is this liquid interior of the sun and stars, 
whose density may vary between say 140 and several hundred 
thousand? As the heaviest matter on earth (the element osmium) 
is only about twenty-two times as heavy as an equal bulk of 
water, and compression does not make much difference, there 


is only one satisfactory answer to this question. The liquid 
matter must consist of atoms which have been more or less com- 
pletely stripped of their outer shells of electrons (see pp. 246-252) 
and so can get closer together on compression. It is these 
outer shells which, as we have seen, give the average atom its 
extreme emptiness, and so endows ordinary matter on earth 
with its ordinary relatively light properties. As we have seen, 
practically the whole of the mass resides in the nuclei, but if 
the nuclei of contiguous atoms are kept at (relatively) vast 
distances apart by the whirling rings of electrons surrounding 
the nuclei, it will be apparent that the most striking thing about 
ordinary matter, as we know it, is its extreme emptiness. But 
if we imagine, under the high temperature influences obtaining 
in the stars, the successive shells to be stripped away, then the 
effect of excessive heat will be to enable the nuclei to get closer 
together, provided that the pressure is sufficient to compensate 
the increased kinetic energy; to get nearer together, at any 
rate, than they ever could get together, whatever the com- 
pression, as neutral atoms fully endowed with their electron 


Now Jeans' theory of electron-stripping is no wild dream; it 
is most abundantly confirmed by experimental and observational 
evidence, so far as a few of the outer ring electrons are concerned 
that is to say, at comparatively low temperatures of a few 
thousand degrees. It is, then, extremely likely that at tempera- 
tures of millions of degrees, which certainly prevail in the 
interior of the sun and stars, this process would be carried 
further, and in some cases to the extreme limit of complete 
stripping of all the electron rings down to the bare nuclei. Then 
we would get the (( dwarf " stars (see p. 327), where matter is so 
dense that a single drop of the liquid might weigh 10 Ibs. Take, 
for example, the atom of uranium, which contains 238 protons in 
a nucleus surrounded by 7 successive electron rings, K-, L-, 
M-, N-, 0-, P-, and Q- (see p. 251). The Q-ring containing 
only i electron is easily shed in the cold, and as the temperature 
rises the 13 electrons of the P-ring are shed, followed by the 
0-ring and so on till we come to the K-ring, the last stronghold 
of 2 electrons, which requires an enormously high temperature 


to be shifted. It is very significant that the stars generally fall 
into groups, which can be identified as having shed successive 
rings down to a given one of the above, but with no stars in 
between. This shows that as each successive ring is stripped 
off, with the rising temperature caused by the annihilation of 
matter, there is a more or less sudden contraction in the size 
of the star to suit the smaller space occupied by the atoms 
newly stripped. The atoms of the giant red stars (see p. 319), 
likeBetelgeuze, are stripped down to their M-rings; others which 
are smaller and yellow are stripped down to their L-rings; 
but the great majority of stars, known as the Main Sequence 
stars, are stripped down to the K-rings, whilst in the massive 
dwarf stars the atoms are mainly present as bare nuclei. Since 
in these dwarfs the electrons cannot reach the nuclei, the latter 
are protected from annihilation, though extremely hot; for 
annihilation involves the mutual disappearance of a proton and 
electron into a violent splash of radiant energy (see p. 269). 
Our sun belongs to the Main Sequence, but its constitution is 
such that it seems to be disconcertingly near the point at which 
the last (K-) ring is stripped off, prior to transition to a dwarf. 
When this happens it will shrink in size to a quite tiny object 
as seen from the earth, and as it will then emit only -j-fo- of its 
present radiation, the sun would no longer be any use to us 
all life on earth would cease, the oceans would freeze and the 
air liquefy. But although the sun is in this dangerous condition, 
there is no need for us to worry; so great are time magnitudes 
in the scale of the universe that Jeans calculates that the sun 
might easily go on as it is for another 2 million million years, 
in spite of the fact that in terms of astronomical measurements 
it is near the verge of changing over. 

The liquid core of the sun, consisting for the most part of 
heavy atoms, exceeding uranium in atomic number and carry- 
ing only the single K-ring, is not spherical but ellipsoidal, owing 
to its relatively rapid rotation compared to that of the visible 
surface. It is the disturbance caused by the protuberant ends 
of this ellipsoid which agitates the gaseous envelope, producing 
the whirlpool storms which we call sun-spots, and makes the 
surface layers rotate more rapidly in the equatorial regions than 


in higher latitudes. The ii-year periodic variation in the 
number and position of the sun-spots may be due to the 
influence of Jupiter, whose period of revolution is about 12 
years, but this is very doubtful. 

Our sun, as we have said, is only a very ordinary kind of 
star with nothing sensational about it. It is simply one of the 
myriad stars forming a compact but stupendous block known 
as the Galaxy (p. 324), which is visible on any clear moonless 
night as the belt, called the Milky Way, encircling the heavens. 
This block, or rather cake, because it is shaped something like 
a thick biscuit, is quite a small piece of the universe; but never- 
theless all the stars we see are contained in it, while their bright- 
ness depends partly on how big they are and partly on how 
far they are off (see p. 329). 

The Planets. After the discovery of Neptune, in 1846, the 
sun's family circle consisted of eight planets, four smaller, 
inner ones viz., Mercury, Venus, Earth and Mars and four 
very much larger, outer ones viz., Jupiter, Saturn, Uranus 
and Neptune. Between the orbits of Mars and Jupiter there 
was an immense gap of over 340 millions of miles, filled with 
a zone of over a thousand asteroids, the smaller members of 
which are still being discovered, and one of which (Eros) 
in its elliptical orbit round the sun sometimes approaches close 
to the earth. The origin of the asteroids is dealt with on 

P* 337- 

During the past fifty years much research has been carried 

out on the planets, for most of which we have to thank the 
Italian astronomer, Schiaparelli. 

Mercury. Schiaparelli, in 1882, stated that Mercury, which 
revolved round the sun in eighty-eight days, had an axial 
rotation of the same period, so that it always presented the 
same face to the sun; one side of the planet was thus in 
perpetual light and the other in perpetual darkness. The 
American astronomer, Lowell, in 1897, affirmed that Mercury 
had no atmosphere, no water and no life "the bleached 
bones of a world/' he called it, and this view is now generally 
accepted. The temperature of the side of the planet facing 
the sun must be about 350 C. i.e., more than enough to melt 


lead; on the other side it must be freezing cold, because there 
is no atmosphere to transmit heat by circulation. 

Venus. Schiaparelli also studied Venus, and believed that, 
as in the case of Mercury, one side was always illuminated while 
the other was always in darkness, for the periods of revolution 
and rotation seemed to be the same viz., 225 days. Other 
observers, however, have stated that there is quick rotation, 
and the temperature observations, which show small differ- 
ences between the illuminated and dark portions of the 
disc, confirm this view. The difficulty is that, owing to its 
cloud-capped atmosphere and the rare appearance of any 
markings, rotation is not apparent, but the conflicting state- 
ments would be reconciled by the hypothesis of Pickering 
(1921) that the period is about three days, with the axis of 
rotation almost lying in the plane of the ecliptic, and not 
approximately vertical as with other planets. The planet has 
a dense cloudy atmosphere, although the presence of water 
vapour is regarded as doubtful. The temperature of Venus, 
according to the Mount Wilson authorities, is just about freezing- 
point, which is somewhat remarkable seeing that Venus is so 
much nearer the sun than Mars, where the temperature is much 
higher. As the temperature observed, however, is no doubt 
that of an upper atmosphere, whereas that of Mars is a solid- 
surface temperature, the discrepancy is intelligible. In the 
case of the earth the upper air is colder than the surface air, 
as we shall see later (p. 369). The temperature in fact falls 
with increase of height for the first few miles (the so-called 
troposphere) and then remains more or less steady in the 
layers above (the stratosphere). The temperature of the 
stratosphere may be 60 C. lower than that at the surface, 
though at 40 miles it is actually higher. 

Mars. Mars is the next planet beyond the earth, and has 
always been an object of special interest. Again, it is to 
Schiaparelli that weowe much of our knowledgeof its topography. 
In 1877 he announced his famous discovery of the " canals * J 
on Mars, more correctly translated " channels/' for " canals " are 
too suggestive of waterways; indeed the American astronomer, 
Pickering, doubted the presence of any water in them. There 


has been much speculation as to the nature of these " canals," 
some going so far as to regard them as artificially constructed 
by Martian engineers ! These fanciful ideas, however, are 
dispelled by the impartial scrutiny of photographs involving 
large telescopes, though it is still uncertain whether the canals 
are real or partly subjective appearances. Mars has a thin but 
distinct atmosphere, and both water vapour and oxygen have 
been identified in it. The equatorial temperature would appear 
to be something like that of a cool bright day on the earth, 
ranging between 45 F. and 65 F. But it must begin to 
freeze at the equator when the sun sets, and at higher latitudes 
even the day temperatures are not much above freezing-point. 
There are thin apparent snow caps at the poles, which can be 
seen to shrink or grow alternately as the summer or winter 
season advances in the respective hemispheres. On the whole, 
it may be said that the existence of life on the planet is quite 
possible at least, we know of no conditions that would render 
it impossible. 

Has Mars a satellite ? Up till 1877 the answer was in the 
negative, but in that year Asaph Hall detected two minute 
squire attendants on the "war planet/' and it says much, not 
only for Hall's powers as an observer, but also for the excellence 
of his instruments, that these bodies were detected at all, for 
Phobos is only thirty-six miles and Deimos ten miles in 
diameter. The existence of these tiny moons was confirmed 
by Lowell in 1894. 

Jupiter,* Turning now to the great planets, What has been 
learnt about them since the middle of the last century? 
Jupiter was the first to claim attention, and, in 1870, the 
English astronomer, Proctor, regarded it as a " still glowing 
mass, fluid probably throughout, still bubbling and seething 
with the intensity of the primeval fires, sending up continually 
enormous masses of cloud to be gathered into bands under the 
influence of the swift rotation of the giant planet/ 1 Jupiter on 
such a view must be regarded as half sun, half world, and this 
interpretation of its structure is that now generally held. 
Jeffreys, 1923, advanced another theory which regarded Jupiter 
as " cold and solid/' but this view has not been received with 


general approval. The most striking features about Jupiter 
are its great cloud belts and the so-called ''red spot/' a large 
area which has been under observation for fifty years. These 
features, however, are not constant, owing to the violent 
turbulence of Jupiter's atmosphere. 

Such turbulence probably betokens a very hot interior, 
causing violent convection and enormously deep clouds, the 
cold tops of which we see as the planet's surface. The tem- 
perature of the latter is certainly very low, about - 170 C. 
This does not necessarily preclude a hot interior (see 
p. 360). 

The likeness to the sun was strengthened when, in 1865, 
Zollner showed that Jupiter rotates more rapidly at the equator 
than towards the poles. Moreover, Jupiter and his satellites, 
in their general relations, constitute an almost complete replica 
of the sun and planets the solar system on a tiny scale. 
Since the time of Galileo it was known that Jupiter had four 
moons, one nearly as large as Mars; but in 1892 a fifth satellite 
was discovered, only 100 miles in diameter, and again, in 
1905-6, four others. This system of satellites resembles the 
planetary system, in the sense that the more massive members 
are found in the middle of the series. 

Saturn, Uranus and Neptune. Saturn also has numerous 
satellites, and eight of these were known before the middle of 
the nineteenth century. The ninth was discovered in 1898 
and a tenth in 1905, although there is still some doubt as to 
the existence of this last. The ring system that puzzled 
Galileo so much has now been resolved into three con- 
centric ribbons of discrete meteoritic particles, each a tiny 

Very little has been added to our knowledge of Uranus, 
which is known to have four relatively small satellites all 
revolving in nearly circular orbits, but in a direction retrograde! 
to that of most planetary objects (which move from west to/ 
east, like the sun itself about its axis). This reverse direction 
of motion is also found with one of the minor satellites of Jupiter 
and one of Saturn. 

Neptune, about which also little is known, has only one, 


relatively large, satellite, but it too moves in a retrograde 

All these outer planets are very light and bulky objects, 
Saturn especially, whose density is only 12 per cent, of that of 
the earth, which is the densest planet . From this it may be sup- 
posed that these giant planets are in a semi-gaseous condition 
that, in fact, they are very hot within, and owing to their large 
size have not cooled down to the same degree as the smaller 
planets. More will be said about this when we come to con- 
sider the earth's atmosphere (pp. 359-3^3)- 

New Planet, Pluto. In 1915 Lowell predicted the ex- 
istence of a small trans-Neptunian planet, which would have 
a mean distance from the sun 43 times that of the earth, and a 
mass 6| times that of the earth; he also predicted its orbital 
eccentricity and position. The calculations were similar to 
those of Le Verrier and Adams, which led to the discovery of 
Neptune, but much more intricate, as the perturbation data 
were on such a small scale. Pickering in 1919 made a similar 
prediction, as well as others, but their conclusions were 
quantitatively different. 

On January 21, 1930, a photograph was taken at the Lowell 
Observatory, Flagstaff, Arizona, which apparently indicated 
the suspected planet, and after seven weeks' observation, on 
March 14 Shapley announced that this object, whose brightness 
was only of the fifteenth magnitude (seep. 329), conformed fairly 
closely to Lowell's hypothetical planet. 

It is, of course, too early as yet (April, 1930) to be certain, 
but if, as it appears, the orbit of Neptune no longer " marks 
the frontier of our solar system " and this is a new planet, 
its discovery will open up an interesting field of work for 
mathematical astronomy. Its insignificant brightness would 
seem to indicate that it is so cold as to have no clouds, and 
if when hot it originally had an atmosphere the gases would 
be in a liquid condition by now, seeing that it must have 
cooled down and can receive practically no heat from the sun. 
Moreover, its distance shows that like Neptune (p. 88) it does 
not conform to Bode's law, for if it did, the distance would be 
about 73 times that of the earth. Further particulars will, of 
course, be awaited with great interest. 


Comets and Meteors. With regard to the other permanent 
or occasional members of our solar system we need say little. 
Meteors and comets are generally acknowledged to belong to one 
and the same category; in fact, a comet " is simply a swarm of 
meteoritic particles more or less closely packed together/' and 
Schiaparelli, in 1873, said that " the meteoric currents are the 
products of the destruction of comets, which is brought about 
by the action of the sun and planets on the different particles 
composing the heads of comets which are thus drawn into 
different orbits/ 1 

The Stars and Nebulae. But advances in our knowledge of 
the members of our own solar system, important as they have 
been, are insignificant as compared with what we have learnt 
about the universe beyond what Lucretius called "The 
glittering ramparts of the world/' Astronomers are now 
armed with instruments that were quite unknown to their pre- 
decessors of last century. The photographic telescope has 
revealed the existence of stars and nebulae in the depths of 
space undreamt of a hundred years ago. Herschel estimated 
that he could see five and a half million stars, but now some 
1,500 millions have been photographed, and yet the limits of 
the universe if it has any limits seem far from having been 
reached. Halley knew of only six nebulae, and Herschel cata- 
logued 2,500; now we know some two million. Again, the 
spectroscope has entirely altered our outlook on the heavens. 
By its means we are able to analyse the chemical nature of 
the faintest star almost as easily as we can some unknown 
substance in a chemical laboratory; we can estimate the 
velocity of its movements, the pressure of its atmosphere at 
different levels, its temperature, as also its distance from us. To 
explain how all this is done, and to give even the merest outline 
of the results obtained, would require far more space than we 
can afford. We must content ourselves with glancing at three 
aspects only of steUar research viz., the distance of the stars 
from us and their size, the amount of heat they radiate, and 
their chemical composition. To give us this information we 
require three instruments over and above the telescope; these 
are the interferometer, the radiometer and the stellar spectro- 


scope. In describing them we will borrow from the excellent 
and beautifully illustrated manuals written by Professor 
G. E. Hale of Mount Wilson observatory, where there are 
to be seen probably the very finest astronomical instruments 
that ingenuity can devise and mechanical skill can produce, 
backed by unlimited monetary resources. 

The Interferometer MICHELSON. So far as the size and 
distances of the stars are concerned, the name that stands out 
most prominently in connection with these subjects is that of 
Albert A. Michelson, who invented the interferometer. The 
following is a condensation of Hale's description of this instru- 
ment given in his book " The New Heavens." 

Make a narrow slit, a few thousandths of an inch wide, in a 
sheet of black paper, and fix it vertically in front of a bright 
light. Observe the slit through a telescope capable of magnify- 
ing about thirty times, placed at a distance of 40 to 50 feet. 
The object glass of the telescope is provided with an opaque 
ca p pierced in the horizontal plane by two circular holes, each 
about | inch in diameter, and each about inch on either side 
of the centre of the cap. When the cap is off, the slit will appear 
as a narrow band with much fainter bands on either side of it; 
when the cap is on, the central band will appear as if ruled with 
narrow vertical lines or " fringes," which are produced by the 
interference of the two pencils of light coming through the 
object glass from the distant slit. Cover one of the holes and 
these fringes at once disappear. Let the two holes in the cap 
be made in movable plates, so that their distance apart can 
be varied. When the holes are gradually separated the fringes 
become less and less distinct, and finally vanish. Measure the 
distance between the holes and divide this amount by the wave- 
length of light, say w ^- ro - inch, and the result is the angular 
width of the distant slit, and knowing the distance of the slit, 
its linear width may at once be calculated. To measure the 
diameter of a star the same procedure is followed, but since 
the angle it subtends is so minute, a very powerful telescope is 
required fitted with a very long interferometer, because the 
smaller the angle the further apart must be the holes over 
the object glass. 


Let us take a definite case. In the southern sky in winter 
there is one very prominent and easily recognised constella- 
tion viz,., Orion. Its essen- 
tial features are indicated in 
Fig. 104, but no detailed de- 
scription need be given, for 
we are concerned with only 
one of its stars, "Betel- 
geuze," whose name is Arabic 
for " Giant's shoulder." The 
interferometer used at Mount 
Wilson was 20 feet long, and 
was mounted on the great 
loo-inch Hooker telescope. 

The plan of the instrument is FlG - IOS.-I 

r ^. ,. , (After HALE.) 

shown at Fig. 103. The light 

from the star, S S, is received by the two movable mirrors, 
i and 4, which correspond to the two holes in the cap of the 
telescope above described. These rays are reflected to the 
mirrors, 2 and 3, and thence to the concave reflector, 5, at the 
bottom of the telescope tube, from which they are again reflected 
to the convex mirror, 6, then to the plane mirror, 7, and thence 
to the eyepiece, E, where the fringes are observed under a 
magnification of 1,500 to 3,000 diameters. On examining 
"Betelgeuze" with this instrument the fringes disappeared 
when the mirrors were 10 feet apart. Without giving the 
actual calculations the result was that the angular diameter 
came out at 0*047 ^ a second of arc, or, to give a homely 
parallel, it was what a ball i inch in diameter would look like at 
a distance of seventy miles ! To know the linear diameter of 
" Betelgeuze," however, we must know its distance, which may 
be estimated from its parallax (p. 74). The parallax was found 
to be 0-02 of a second of arc i.e., the angle subtended by 
the radius of the earth's orbit at the distance of " Betelgeuze." 
This would make the diameter of the giant at least 100 millions 
of miles, or more than 100 times that of our sun, and its 
distance from us at least 160 light years (see p. 320). In other 
words, light travelling at 186,000 miles a second takes 160 years 


to cross the space between "Betelgeuze" and the earth, so 
that the light we see now left the "shoulder of Orion" a few 
years after Prince Charlie landed in Scotland ! 

A "light year" is a convenient astronomical unit of dis- 
tance, and being the distance covered in a year by light travel- 

Aldebaran . l ^ at l86 ' OO 
.,--'"" .' miles a second, so 

Betelgeuze *--""" / is *86,ooox6ox 
,--.. / 60x24x365 miles 

"'^y .' (5-gxio 12 ). "With 

the naked eye," 


writes Professor 

/ Eddington(" Stars 

;, Nebula morion and Atoms ), 

"you can see the 
r^:::~.. ......... R fg e i Andromeda Neb- 

Sf rius ula as a faint patch 

ORION WITH ADJACENT STARS. ol u & nt - wn.en 

you look at it you 

are looking back 900,000 years into the past." The " depths 
of the universe" are truly appalling. 

By its parallax " Sirius " (Fig. 104), the brightest star in the 
heavens, is estimated to be about eight and a half light years 
distant, and there are only three other great stars nearer to us 
than " Sirius." But even " Betelgeuze " is a near neighbour 
as compared with some of the 1,500 million stars photo- 
graphically observed, and many nebulae are known to be as much 
as millions of light years away ! A better idea of these 
distances will be gained later when we consider stellar evolu- 
tion and astronomical space and time (p. 338). 

The Thermocouple and the Radiometer HUGGINS, 
LEBEDEW, NICHOLS. To be able to measure the size and distance 
of a star is indeed a great feat, but to estimate the heat it gives 
out would seem an absolute impossibility and yet it has been 
done. Sir William Huggins (1824-1910) was the first to make 
the attempt. He was a pioneer in astro-physics by bringing 
into use, in astronomy, all suitable appliances from the physical 
laboratory. One of these was the thermocouple, a conjunction 


of two metals which were very sensitive to radiant heat. The 
principle of this apparatus will be understood from Fig. 93. 
Solder together an iron and a copper wire, and connect the 
free ends with a galvanometer (Fig. 105, G). When heat is 
applied to the junction of the wires, the needle of the galvano- 
meter at once shows a deflection, indicating that a current is 
passing through the wires, and the amount of the deflection 
shows its strength and indirectly the degree of heat affecting 
the couple. Huggins, in 1869, attached such a couple to his 
8-inch telescope, but although he thought he got an effect, his 
results were not confirmed. In 1895 the Russian physicist, 
Lebedew, invented a much more sensitive thermocouple made 
of iron and const antan (an alloy of copper and nickel), with 
which positive results were obtained a few years afterwards. 
Complete success, however, rewarded the use of a new radio- 
meter, invented in 1898 by the American physicist, Nichols. 
This instrument consisted of 
exceedingly delicate vanes of 
mica suspended in a vacuum. 
These vanes received the stellar 
rays reflected from a 24-inch 
concave mirror, the degree of 
deflection being observed FlG . I05 _ A SlMPLE THERMO ~ UPLE . 
through a small auxiliary tele- 
scope. Hale gives the following illustration of the sensitiveness 
of the apparatus. It was found that the average deflection of 
the vanes induced by a candle 2,000 feet distant was 67 mm. 
Nichols's assistant, who was in charge of the light, blew it out 
at a given signal, and inserted his own head in its place, when 
the deflection given by the radiometer was 25 mm. This 
experiment was tried again and again, so that blood-heat, 
even after absorption by the intervening air, could be registered 
at a distance of nearly half a mile ! 

When the radiometer was used along with the loo-inch 
Hooker telescope at Mount Wilson, it was found possible to get 
quite astonishing results. The average deflection given by 
the star " Arcturus," whose diameter is 21 millions of miles 
and whose distance from us is estimated to be not less than 



thirty light years was ro8 mm. Thousands of observations 
of this kind have been made during the past few years, both 
with Nichols's radiometer and also with greatly improved 
thermocouples, and from the results obtained astronomers tell 
us that " the redder the star the greater the proportion of in- 
visible heat radiation it sends us/' A red sun like " Betel- 
geuze" has a surface temperature of 2,500 to 3,000 C., 
and its density is exceedingly low, but in the centre the 
pressure is higher, and the temperature is as high as 2 to 
3 million degrees. Such a star radiates out heat on a gigantic 
scale, but it will decrease in diameter and waste slowly away, 
while the temperature will imperceptibly rise as the star 
changes from red to yellow and then to white. The surface 
temperature of a white star may exceed 20,000 C., and its 
central temperature may exceed 100,000,000 C. ! In the 
heavens we have presented to our gaze every variety of stage 
in stellar evolution, as we shall see presently; and in this 
orderly sequence, or, as it is sometimes called, Celestial or In- 
organic Evolution, we thus have giant red stars at one end 
\of the scale and dwarf white or red ones at the other. Our 
town sun, as we have seen, is on the way to become a dwarf 
star and has a surface temperature of about 6,000 C. with a 
central temperature of approximately 30,000,000 C. (Hale), 
or 40,000,000 C., according to Eddington. 

All stars by radiating energy copiously must be losing mass, 
matter being as we have seen (p. 269) a stupendously con- 
centrated form of potential energy. So according to the latest 
theory of Cosmogony put forward by Jeans, the sun and indeed 
most of the stars are mere fragments of the massive giants they 
once were. And so he explains the sun's continued output of 
energy, maintained over millions of millions of years, by the 
annihilation of material atoms and transformation of matter 
into radiation. 

The Spectroscope and Spectrum Analysis. When, in 
1868, Lockyer discovered the element helium in the sun, he 
opened up an entirely new chapter in astronomy viz., the 
chemical analysis of the stars by means of the spectroscope. We 
have already seen (p. 164) how helium was isolated by Ramsay 


from a terrestrial mineral nearly thirty years afterwards, 
and the extraordinary part of the story is that this new element 
should have been found in a stellar body 93 millions of miles 
away, long before it was discovered at our own doors. 

During the past fifty years our knowledge of the chemistry 
of the stars and nebulae has been vastly extended, and, while 
no strange elements have been discovered (unless we include 
the hypothetical " coronium " in the solar corona), many of the 
terrestrial elements appear to be missing in several of the stars 
that have been analysed. To take the case of our own sun, 
the following elements have been identified in it by the aid of 
the spectroscope: hydrogen, oxygen, calcium, iron, sodium, 
carbon, magnesium, cobalt, aluminium, chromium, strontium, 
manganese, copper, zinc, cadmium, silver, tin, lead, potassium 
and others; but, strange to say, some elements of immense im- 
portance to us on the earth are apparently entirely absent, such 
as sulphur, phosphorus, mercury, gold and nitrogen. Still, it is 
possible that by adjusting the intensity of the spectrum some 
of these may yet be found. We know that the bright lines of 
sodium vapour may be made so intensely bright that the 
spectrum of limelight placed behind the vapour does not 
" reverse " or turn them into dark lines. If the sodium lines 
be made fainter they may be reduced to exactly the intensity 
prevailing in that part of the spectrum of limelight, in which 
case the lines would not be distinguishable at all. 

There is another use to which the spectroscope has been 
put in relation to stellar exploration. If we are approaching 
a source of light the waves will meet us more quickly than if 
we are stationary, or more slowly if we are receding from it. 
When we are approaching, the waves will appear shorter, and 
this is indicated by a very slight shift of the lines of the spectrum 
towards the violet end. Conversely, if we are receding, the lines 
will be shifted towards the red (Doppler effect, see p . 307) , On ex- 
amining " Sinus," we find that same Hne, F, of the spectrum is 
slightly displaced towards the red end, and from the amount of 
that displacement we are able to deduce that Sirius is receding 
from the solar system at the rate of about 70,000 miles an 
hour. " Arcturus," on the other hand, is approaching at about 


200,000 miles an hour; but seeing that " Arcturus J> is thirty 
light years away we need not feel alarmed, for there is no 
possibility of a collision for at least a couple of million years, 
even if the speed and direction towards us were maintained. 

Stellar Evolution and Modern Cosmogony Great as 
are the achievements of observational astronomy, they pale 
before the greater triumphs of the human mind in solving 
the riddle of the universe by mathematical analysis of the 
observational results. The names of Einstein, Eddington, and 
Jeans will ever be associated with these mathematical triumphs ; 
but here, of course, it will only be possible to give the broad 
outlines of the picture without details. The world owes a 
debt of gratitude to Jeans especially, and we must now draw 
freely on his fascinating books (p. 309) to render his conclusions 
intelligible to enquiring minds. Now the power of Mathe- 
matics, especially the calculus, in the service of Science is so 
wonderful in elucidating what would be otherwise insoluble 
problems, as to be almost uncanny and scarcely believable 
to those unacquainted with its intricacies. Every branch of 
astronomy, involving the interrelationships between time, 
space, matter, and energy has been submitted by Jeans to such 
mathematical analysis, and the infinitely complex results 
have been integrated into a relatively simple picture portraying 
the Cosmos. By five or six independent methods of approach 
the age of the Galaxy (p. 312), including our sun, has been 
determined and found to be about 5 to 6 million million 
years, and prior to this it existed as one vast rotating gaseous 
nebula, similar in almost every way to the 2 million nebula 
(outside our system) which are scattered in every direction 
through the known universe. 

This single nebula, which gave birth to the galactic system 
that we call the Milky Way, as a sort of small universe on its 
own, can no more be explained than can any other nebulae; 
but the changes it must have undergone in its history can be 
calculated, and they are found to be similar to what is happen- 
ing now to the extra-galactic nebulae. Briefly speaking, the 
sequence of changes involved were (i) contraction with increas- 
ing speed of rotation, and consequent flattening from an 


approximately spherical shape to that of a lens, (2) eventual 
instability when the speed of rotation exceeded a certain 
critical limit, gaseous matter being then ejected from the sharp 
edge in the form of two oppositely situated equatorial arms, 
as a kind of spiral. 

This spilling out of incandescent gaseous matter by centri- 
fugal effect was, of course, on a stupendously vast scale, 
comparable in dimensions to that of the Galaxy as we know 
it now (see below for dimensions), but otherwise similar to that 
conceived by Laplace (p. 84) on an infinitely smaller scale for 
the origin of the solar system. 

Ultimately the whole nebula was dispersed in this way, 
and the shed equatorial matter became subject to what is 
called gravitational instability, which led to the formation of 
centresof condensation, as vast and heated globes of gas, destined 
to become independent stars by further shrinkage. Stars, then, 
are to be regarded as " drops " or points of condensation, which 
have formed in the j ets or filaments thus thrown off horizontally 
from the edge of the shrinking lens or disc, giving the present 
plane, in fact, of the Milky Way. 

The number of such stars, so born out of our parent nebula 
forming the present galactic system, amounts to anything 
from 30,000 million (Seares) to 300,000 million (Eddington), 
though Shapley gives a figure of 100,000 million. 

Herschel and (later) Kapteyn were wrong in supposing our 
sun to be near the centre of the system, and Shapley believes 
the centre to lie in a massive star-cloud in the constellations 
of Scorpio and Ophiuchus, 47,000 light-years away from the 
sun. The confusion has arisen from the fact that near the 
sun there is a "local system " or great cluster of stars Hke 
another galaxy on a smaller scale, lying in a plane inclined 
12 to the galactic plane and not in it. This local system is 
in the form of a relatively small flattened disc, or rather cake, 
like the great biscuit-like shape of the whole galactic system, 
which is roughly 220,000 light-years in diameter and 40,000 
light-years thick. The sun, which till recently was believed 
to be only about 2,000 light-years from the centre of the 
Galaxy, is now believed to be about twenty times this distance 


away, slightly to the south of the horizontal plane of the 
Milky Way. 

The movements of the multitudinous stars in this immense 
system were first detected by Kapteyn in 1905, in the form of 
two distinct star-drifts, and these movements have been very 
difficult to disentangle. But, generally speaking, it has been 
found that in addition to a lot of independent motion of groups 
of stars and individual clusters like the Great Bear, Pleiades, 
etc., there is a general movement of rotation of the system as a 
whole round the centre, one revolution occupying some 300 
million years. This conclusion is based mainly on the work of 
Oort and of Plaskett ; and it is interesting by way of comparison 
to observe that the period of revolution of the Great Andromeda 
Nebula is of the order of 19 million years. 

This relatively near nebula seems to be a counterpart of our 
own galactic system, of which it is independent, at an immense 
distance away (about 900,000 light-years) ; but it is at an earlier 
stage of evolution namely, at the stage in which stars are being 
born in its outside regions, while the centre is still in a gaseous 
condition. It has a distinctly spiral shape, illustrating how 
matter is being flung off at the equatorial regions ; it is flattened ; 
and its total mass, according to Hubble, is equivalent to 3,500 
million suns. There are, as already stated, something like 
2 million such nebulae (though not so near and conspicuous as the 
one in Andromeda) scattered throughout the visible universe, 
and they show every gradation in the evolutionary sequence, 
from spherical gaseous masses, through flattened figures spilling 
out equatorical matter, to nearly finished configurations of 
stars like " island universes/' similar to our own Galaxy. 

Turning to the individual stars of our own system, we find 
they are much more crowded together near the centre (though 
relatively at enormous distances apart from each other, 
on average), but thin out considerably as we approach 
the outer confines of the Galaxy. The near stars, which are 
mostly visible to the naked eye, and amount only to a few 
thousand, belong, of course, to our own system, as do the vast 
numbers revealed by telescopic photography. Among these 
there is immense variety as regards size, temperature, and 



luminosity, but not such great variety in mass. Most of them, 
indeed, are somewhere about the sun's mass, and for the vast 
majority the extremes are between & -and ten times the sun's 
mass. It is true that there are a few stars, like Plaskett's 
star, sixty or seventy times the weight of the sun, or even more, 
but the variation in size and luminosity is much more extreme. 
The diameter of Antares, the largest known, is 450 times 
that of the sun, and van Maanen's star, the smallest known, 
less than T ^ that of the sun, so that the latter is actually 
smaller than the earth. But this dwarf has a surface tempera- 
ture of 7,000 (i.e., greater than that of the sun), and its mass 
appears to be about that of the sun, so that it must have an 
enormous density, the reason for which has already been 
considered (p. 310). 

In respect of luminosity, the greatest known is that of 
S. Doradus in the MageHanic Cloud, 300,000 times that of the 
sun, sufficient indeed, if it were in the sun's position, not only 
to scorch the earth but to raise its temperature to 7,000 C. 
and so convert it into a globe of gas. At the other extreme is 
the star Wolf 359, whose luminosity is only ^fa* that of the sun. 

In general, however, most of the stars must be regarded as 
very similar to the sun, consisting, like it, of an internal liquid 
core of atoms, more or less completely stripped of electrons, 
and an outer gaseous envelope, the surface of which, only, lends 
itself to observation. The liquid core of some stars must have 
a rapid speed of rotation, and it is possible to calculate what 
will happen when this speed increases, as it must by contraction 
following the stripping off of successive rings of atomic electrons, 
as already explained. 

Binary Stars. The behaviour, then, is very different from 
what it is with a rotating mass of gas like a nebula, where most 
of the mass is concentrated towards the centre. Instead of de- 
veloping a flattened or lens-shaped form, which is calculated 
for such a case, and which must spill out equatorial matter from 
its edge by any increase in rotation, a spherical rotating liquid 
mass, whose speed of rotation increases, must pass through a 
different sequence of changes in form. This sequence commences 
in a flattened or orange shape (pseudo-spheroid), and by further 


inc^ased speed develops into a pseudo-ellipsoid form, which is 
AH elongated shape having three unequal axes. When the 
latter reach the critical ratio 23 : 10 : 8 increased speed of 
rotation lengthens it to a cigar shape, then develops a furrow 
or waist, somewhere towards the middle, when the length is 
about three times the width. After this the mass begins to con- 
centrate at each end, something like a dumb-bell, the furrow 
deepens till it finally cuts the body into two detached, nearly 
spherical, masses which then continue to rotate about the 
common centre of gravity, lying on a line joining their centres. 
As a rule, the two masses are not equal, but they are more 
nearly so than in the case of the earth-moon system. 

In this way binary stars have been formed, and as time 
elapses they get further and further apart, owing to influences 
which we need not consider here, but they still continue to 
revolve around their common centre of gravity. Those stars 
which have recently suffered fission in this way cannot be 
distinguished as separate objects in the telescope, but they 
may, by getting in each other's way, cut off periodically a 
certain amount of light we receive from them, and thus appear 
as variable stars (eclipsing binaries) like Algol in the constella- 
tion of Perseus. Or they may be detected by the fact that 
while one is approaching us in the line of sight, the other is 
receding, and this (Doppler effect, see p. 323) may be detected 
in the colour of the spectral lines (spectroscopic binaries). 

Those binaries which are not too distant from us, or which 
have moved far apart, maybe detected easily by the telescope 
and their mutual orbits traced out ; thus the pair of a-Centauri 
(A and B), which is the nearest binary, has a period of about 
seventy years, and a complete orbital revolution has already 
been mapped out. 

Cepheids. According to Jeans, the so-called Cepheid 
variables, which are very peculiar in their spectroscopic be- 
haviour, are stars whose liquid cores rotate so rapidly that they 
are just on the verge of fission, but this view is not as yet 
generally accepted. His theory is that the liquid core ploughs 
round and round at such a speed as to throw up a steep and 
moving wall or wave of hot gaseous matter, in the surrounding 


envelope, at short regular intervals. The speed of this moving 
wave, which occasions the periodicity in light-fluctuation, must 
be prodigious, since the period is only of the order of a few 
hours or days, and as the Cepheids are very large stars this 
would mean a condition bordering on fission. Whether this 
theory is accepted or not, it is certain that the periodicity in the 
luminosity is somehow connected with the brightness, that 
is to say with the real brightness or so-called "absolute, 
magnitude/ 1 It must not be forgotten that the ordinary or 
so-called " visual magnitude " must depend on distance as well 
as real intrinsic brightness, and for two stars of equal brightness 
(i.e., identical absolute magnitude) if one were twice the distance 
of the other its luminosity would be only one quarter (law of 
inverse squares, which holds for all radiation), and so the 
visual magnitude would be much higher.* 

In 1912 Miss Leavitt of Harvard found that in all cases the 
visually brighter Cepheids fluctuated more slowly than the 
fainter ones. Now since the distance of the nearer ones could 
be measured by the parallax method, this fact could be utilised 
for measuring the distance of clusters containing Cepheids 
beyond the limits of the parallax method, by building up as 
it were a measuring-rod and step by step plumbing farther and 
farther into the depths of the universe viz., on the basis that 
equal period means equal intrinsic brightness, and so the 
observed visual magnitude gives the distance by the law of 
inverse squares. 

In the hands of Herzsprung and Shapley this elegant method 
has opened up vistas of such immense distances that it is at 
last possible to construct a rough sort of model of the universe 

* As bright stars are said to be of the first magnitude (visual), it follows 
that those stars of second magnitude, etc., are not so bright. Stars of the 
fifth and sixth magnitude are very faint to the naked eye. As a matter of fact, 
the brightness increases two and a half times for every step down in magnitude, 
so that in the five steps from the sixth magnitude to the first there is an increase 
in brightness of no less than 100 times. In astronomy, fractions of a magni- 
tude are employed, also minus numbers to indicate very great brightness; 
and the " absolute magnitude " of any star represents its luminosity as it 
would appear at a standard distance of 32-6 light-years. Thus, for example, 
S. Doradus, the brightest known star, has Mag. a b S -9, Sirius 1-3, and the 
Sun 4*85, 


itself (see p. 340), Returning to the Galaxy, we observe that 
whilst the vast majority of stars must have followed an 
evolutionary course similar to the sun, some 20 or 30 per cent, 
were endowed with a higher speed of rotation and so have 
gone over into double stars. But all of them are much about 
the same as our sun in age (p. 309), though vastly varying in 
luminosity and size; and the only satisfactory explanation 
of this long continued output of radiation is found in Jean's 
theory of annihilation of hypothetical types of atoms, higher in 
atomic number than uranium (see p. 309). 

In its early history a star was simply a distended gas balloon, 
as some of them still are. In this state, as Eddington has 
worked out, they would be subject to the forces of radiation 
pressure in addition to gravitation, this pressure, negligible with 
ordinary hot bodies, being of the same order as gravitation at 
the temperatures in question. Eddington believes this was 
the principal force at work in causing independent centres of 
condensation (potential stars) to appear, but Jeans finds that 
gravitational instability (p. 325) would be sufficient to bring 
the segregation here, as well as in the formation of the solar 
system itself, as we shall see later. In any case the gas globes 
would tend to shrink, but this tendency was, on Jeans 1 
annihilation theory, checked by the great initial output of 
radiative energy, and on this theory the prime cause of 
shrinkage was the stripping of electronic rings, as the interior 
temperature rose by reason of this great output exceeding 
the loss by radiation at the surface. The evolution, in short, 
was similar in the majority of cases to that of the sun 
already outlined (pp. 309-311). 

All stars were thus, like the sun, originally more massive 
and more prodigal in their youthful and wasteful expenditure 
of energy than they now are, in their old age so to speak; 
and on this theory it would seem as if the few massive and 
brilliant giants which still exist, and which may show thousands 
of times the energy-expenditure of the sun, must be young. 
If so it is difficult, on the annihilation theory, to account for them 
as having been born out of the original nebula, along with the 
majority of the stars. It is possible that they are not young, 


but that the matter within them has in some way been preserved 
from annihilation, in the intervening ages, say by segregation 
of the nuclei apart from the electrons. Of course, this is purely 
speculative, but it finds some support in the existence of the 
so-called " white dwarfs," which, though at an enormous 
temperature within, are in some way immune from sub-atomic 

Russell, whose brilliant work on the classification of stars 
has been of such service in developing these new ideas, does not 
altogether accept them. The stars are classified according to 
spectral type, which is really that of surface temperature in the 
series: 0, B, A, F, G, K, M, with divisions i to 10 between each 
letter, representing a type; the 0-stars being the hottest (say 
30,000 C. at the surface) and M-stars the coolest (about 2,500 
C.) Russell's first idea was that M-stars with time got hotter 
by contraction and passed into the K, G, F, and A types 
successively, and then by cooling returned along the same 
sequence in the reverse direction. This theory of an ascending 
and descending scale of temperature, with the progress of time, 
was originally put forward by Sir Norman Lockyer towards the 
close of the nineteenth century in his book "The Meteoritic 
Hypothesis," but owing to difficulties which need not be con- 
sidered here the theory cannot be accepted in its original form 
i.e., contraction causing temperature increase. In 1925 Russell 
modified his first views and put forward the theory that when 
the centres of stars reached a critical temperature of 30 million 
degrees C. by contraction, matter itself became unstable and 
annihilated itself with enormous generation of energy. As 
a result of this great increase in energy the temperature 
tended to rise above the limit of 30 million degrees C., but by 
so doing expanded the volume of the star, and so cooled it 
(adiabatically) below this point of material disintegration. 
Then, after a time, cooling and contraction would supervene and 
the cycle repeat itself* 

It is true that rhythmic contraction and expansion may 
take place in some stars, and may account for some of the vari- 
ables which cannot be explained otherwise. Betelgeuze (p. 322), 
for example, shows changes of no less than 25 per cent, of its 


diameter at regular intervals . Nevertheless there are difficulties 
of a mathematical kind in the way of accepting Russell's theory, 
as Jeans has pointed out; but the latter mainly objects to it 
on the ground that if matter became unstable at some particular 
or critical temperature, any star where this temperature was 
attained would become so violently unstable as to resemble 
gunpowder at its flash point. Here we must leave the problem 
and pass on to consider our own solar system. 

Origin of the Solar System. The original Nebular Hypo- 
thesis, put forward by Laplace (p. 84), was later found, on 
mathematical examination, to fail to explain the existence of 
our planetary system, among other reasons, because the centri- 
fugal effect, due to the angular momentum, of the rotating 
"nebula" from which the system was supposed to have 
developed, could never have been sufficient to throw off the 
planets and their satellites. 

Thus the angular momentum of the whole solar system 
(over 95 per cent, of which is due to Jupiter) is only about 
twenty-eight times that of the sun, and if it were put back 
into the sun the latter would rotate at a speed of about once in 
24 hours, which is a much lower speed than that of Jupiter 
(about 10 hours), which holds together. On the other hand, if 
the sun functioned as a gas with most of its mass concentrated 
in the centre, it is possible with a high speed of rotation to 
assume the lens-shaped figure which we saw (p. 325) preceded 
the equatorial disruption of nebulae, but in this case it can be 
calculated that 85 per cent, of the mass would have to be at 
the centre. Now, of course, this seems impossible on the older 
views as to the sun's constitution, but on Jeans' theory of 
"liquid stars" it is conceivable; but even so, by a theorem 
of Poincar^ it can be shown that ejected matter from the 
equatorial edge of a lens-shaped figure would have to possess 
a higher density than is possible for the planets, if it were 
not to scatter away into space by its own internal pressure. 
Further, there would be the difficulty of explaining how the 
sun ever got back from such a figure to spherical, how the 
satellites could have formed from scattered matter, as well 
as other difficulties. 


Towards the end of the nineteenth century Lockyer put 
forward the Meteoritic Hypothesis, by which the stars and 
the solar system were supposed to have been evolved from the 
gravitational clashing of small cosmic particles or meteorites, 
which are certainly very numerous in space; but this view 
meets with grave difficulties on close examination and has 
not been accepted. 

A more recent attempt is the Planetisimal Hypothesis of 
Chamberlin and Moulton, in which the tidal effect on our sun 
is invoked, raised by a star which at one time passed very 
close to it. There is no doubt that the enormous tides which 
would be so raised, on a rotating ball of practically gaseous 
matter, could cause such disturbances as to produce eruptions 
of superficial material on the side nearest the star, as well as 
on the opposite side. On this hypothesis, this ejected matter, 
in the form of fine particles or planetisimals, revolved round 
the sun in many orbits and eventually coalesced into a com- 
paratively few globes of matter which are now planets and 

A modification of this hypothesis has recently (1924) been 
put forward by Jeans, who has mathematically examined the 
whole problem of planetary evolution in a most exhaustive 
manner, taking into account all the known facts elucidated 
by modern research. Let us consider this hypothesis, 

We have seen how millions of stars can be born out of a single 
gaseous nebula whose mass is mainly concentrated in the centre. 
The sequence of shapes arising from increase in rotation, with 
such gaseous masses, is very different from what it is in the 
case of rotating liquids (seep. 327); the calculated sequence for 
gaseous nebulae begins, as with liquids, with a spherical followed 
by a flattened spheroidal figure, but this later assumes a flat- 
tened lenticular shape with a sharp equatorial edge* When 
this critical figure is reached any further increase in speed of 
rotation must lead to instability, and it is this instability that 
causes the shedding of filaments of matter, spilling out as the 
spiral equatorial arms as we have seen on p. 325, and giving* 
birth to stars by local condensations. Moreover, such local 
condensations, denoting stars or clusters of stars in process of 


birth, are telescopicaUy visible in the nearer extra-galactic 
nebulae, and every stage in the evolutionary process, from 
spherical to spiral nebula* in process of disruption, has been 
observed in different parts of the universe. Each of these 
numerous nebulas is comparable in mass to that of our own 
galactic system, which it is supposed has already gone through 
such an evolutionary process about 5 to 6 million million 

years ago. 

Up to this point mathematical theory and observational 
astronomy are in harmony. Difficulty only arises when the 
further evolution of each independent star is considered; for 
mathematical investigation shows that, instead of planetary 
systems being developed, the normal course is for the star 
either to remain a single spheroid or to split into two (binary 
stars), as the star contracts and thereby increases its speed of 
rotation. In no case does it appear possible for matter to 
be detached equatorially and form planets on the lines of 
our solar system, and so there must be something rather 
exceptional about our sun and his family of planets. At this 
point Jeans admits that exact science ends and speculation 
begins, but he postulates that the only conceivable explanation 
of the anomaly is the chance encounter, at one time, of our 
sun with another star. With the present distribution of the 
stars in space, the probability of such an encounter is too 
remote to consider seriously. Our nearest neighbour at 
present, for instance, Proxima Centauri, a tiny but massive 
dwarf near the a-Centauri pair (p. 328), is twenty-four million 
million miles away, and even if it were only a small fraction 
of this distance away its tidal influence on the sun would be 
negligible. And the theory of probability shows that since 
the beginning of things insufficient time has elapsed (say within 
the last six million million years, which is approximately the 
sun's age), for such a star to come within the critical distance 
necessary to pull tidal matter out of the sun. But this is sup- 
posing the present average distance of the stars of the Galaxy 
has not sensibly altered, and the difficulty would be met if 
it were assumed that, at one time, the concentration of stars in 
space near our sun was considerably greater than now. There 


is a priori probability for this assumption, with its increased 
chance of an encounter by which such a cataclysm might have 
happened. That is to say, some neighbouring star, wandering 
by, came very close to our sun, not close enough for a collision 
or even close enough to interfere with the independent career of 
each, but nevertheless within that critical limit (which can be 
calculated for different sizes and masses) necessary to pull 
tidal matter out of our sun. This distance must have been 
something between one and two solar diameters, and the 
wandering star must have performed a curved journey (owing 
to mutual gravitational attraction) towards and from the sun, 
in a plane inclined some 6 or 7 to the sun's equator. This 
plane, now the plane of the ecliptic, is that in which the main 
planetary mass now moves round the sun, the latter having con- 
tinued to rotate round the same axis as before the cataclysm. 
There seems to be every reason to believe that this must have 
all happened within a short space of time, about two to three 
thousand million years ago. This, of course, is but yesterday 
compared with the great age of the sun, and in the encounter 
with the stranger, it is just as if our solar mother had mated, 
after a barren life of fifty years, and only a week ago given 
birth to a family of lusty planets in a single litter. It is now 
possible to picture how this birth, which is so recent an event 
in astronomical time, took place. 

As the star approached nearer and nearer, the solar tides grew 
ever higher and higher until at last when the critical distance 
was reached, the tidal matter ceased to fall back completely, 
the sun's gravitational attraction being just balanced by the 
centrifugal force and star's attraction. As the star got nearer 
a jet of half-liquid, half-gaseous matter streamed out from 
near the sun's equator. This jet or filament, like a moving 
spiral, increased in amount and thickness as it was payed out, 
being fully gaseous during near approach, but it diminished in 
amount and soon became mainly liquid as the disturbing star 
passed away. This stupendous torpedo-shaped coil of matter, 
massive but gaseous in its middle, was meantime forming its own 
centres of condensation and rotation, and later at most centres 
a similar process repeated itself on a diminutive scale, thus 


eventually giving rise to the satellites, circling round their 
primaries, and revolving in the same direction. 

The exact mode of these condensations into potential planets 
and satellites is beyond thepower of mathematics to unravel. But 
the first condensations must have occurred, according to Jeans, 
almost at once, owing to gravitational instability (see p. 325) 
while they were under the influence of both sun and stranger, 
and were still for the most part very hot and gaseous. 
Jupiter and Saturn must have reproduced the solar cataclysm 
almost exactly, but, of course, on an immensely smaller scale; 
but their satellites did not repeat the process, because they 
were so small as to cool down quickly and so become liquid. 
And so there was a limit to the birth process when grandchildren 
were born. 

At the thin ends of the original filament the rate of cooling 
must have been very rapid, and so the condensation centres 
here quickly became liquid planets . The disruption of spherical 
liquids under tidal influence follows a very different course 
from that shown by gases. It can be shown mathematically 
that excessive tidal forces first elongate the sphere to a long 
spheroid, which then furrows at one end, giving a pear-shaped 
figure. This finally develops a bulb at the narrow end, and 
suffers fission, giving a detached mass (satellite) which is rela- 
tively much more massive in comparison to the total weight, 
than that in the case of tidal disruption of gaseous figures. 

It is very probable that some tidal influences causing the 
detachment of satellites from their primaries were induced by 
our own sun after the stranger had passed. For the orbits 
of these primaries, or potential planets, would be exact ellipses, 
and there would be so much mutual disturbance that these 
primaries may in their early existence have come very 
close to the sun in perihelion passage. But the whole 
regiofi of the solar system must have been so full of scattered 
debris and light gases, which had escaped, the planets having 
to plough their way through this resisting matter, that their 
orbits ^ould gradually become more circular. In any case 
most of this scattered debris must have been picked up by the 
planets, the rest having escaped into outer space; but the 


zodiacal light in the neighbourhood of the sun shows that there 
is some of this cataclysmic material still left as witness of the 
great event of those far-off days. 

At all events, when the wandering star had passed away, all 
this incandescent matter, no longer subject to outside interfer- 
ence, settled down to form globes and satellites, now subject 
to the sun's influence, and cooling down to form the planets, 
etc., as we know them today. Those like Neptune and the 
earth which were born more or less liquid have, as they ought 
to, few (one each) but large satellites, those like Jupiter and 
Saturn which were born wholly gaseous have relatively many 
but small satellites (because they were born bulky). Mars 
was born gaseous, but being towards the thin extremity of the 
gaseous filament it was not massive enough to hold itself to- 
gether; that is to say, because the kinetic energy of the gaseous 
matter at high temperature (p. 224) was superior to the gravita- 
tional potential most of it has been dissipated off into space, 
and the same is true (for the same reason) with Uranus, which 
was originally much larger than it now is. The inner planets 
Venus and Mercury were born liquid at the thin extremity of 
the filament, and they were apparently unable to cast off 

It only now remains to consider the Asteroids and Saturn's 
rings. These offer no difficulty when it is considered that 
tidal influence, being the effect of differential gravitational 
attraction on the opposite sides of any mass, becomes relatively 
very great when the attracting objects are near. There is a 
critical distance known as " Roche's limit/' below which any 
large solid mass like a satellite approaching its primary must 
be torn up and broken into fragments by the differential forces 
at work. Roche's limit is a distance from the surface of the 
primary about twice to three times the radius of the latter, 
depending on the density of each object. For example, if the 
moon approached to within 12,000 miles of the earth (as it 
will do eventually, if nothing intervenes to prevent its 
present recession to its limit in 50,000 million years followed 
afterwards by slow approach to the earth), it would be broken 
up into a mass of minute fragments which would then circle 


independently round the earth as a single ring, something like 
Saturn's. The innermost satellite of Jupiter is already danger- 
ously close to the Roche limit and may break up at any time. 
The innermost satellite of Saturn has already done so (probably 
in the early days when there was more irregularity), splitting 
into three rings instead of one, because of the influence of the 
other satellites, which make the spaces between the rings an 
unstable orbit for any satellite. 

And the Asteroids probably represent a planet which has 
been disrupted in this way, owing to the fact that in its early 
history, when planetary orbits were more eccentric than they 
are now, this planet wandered into the danger zone of the 
Roche's limit of Jupiter (or the sun) and so met its untimely end. 

Also, according to Jeans, the new trans-Neptunian planet 
(p. 316) may represent the extreme tip of the original cigar- 
shaped filament and thus be the first to cool down and solidify; 
' ' as a consequence of this it will probably prove to be unattended 
by satellites/' Such in brief is the bold and daring concep- 
tion envisaged by Jeans to explain the existence of the solar 
system. It is not susceptible of scientific proof, but, as he 
says, with this conception "the pieces of the puzzle begin to fit 
together in a very gratifying manner/' 

Astronomical Space and Time. In dealing with space on 
the grand scale of the universe difficulties are encountered 
which have presented themselves to all philosophers, indeed, 
to everyone who thinks and tries to imagine where space ends 
or when time began. These difficulties arise mainly from a 
sort of instinctive feeling in the human mind that space must 
go on for ever, and that time is an independent thing which 
flows on for ever. This, the Newtonian system of philosophy, 
received a rude shock when Einstein introduced his conception 
of relativity, which, as we have seen, identified time and space 
in one comprehensive continuum, in which a mathematical 
time-function appears as a fourth dimension, inseparably 
bound up with the three dimensions or directions (length, 
breadth, and height) of distance, dimensions which we ordinarily 
keep separate in our minds and call space. 

According to relativity this separation is quite artificial, 


even if it is convenient in dealing with the things of ordinary 
experience, but it breaks down completely when astronomical 
magnitudes are in question. Relativity, the theory of which has 
received experimental verification on every issue that can be 
so tested, tells us that astronomical space-time is not infinite 
but rather unbounded. It is so in the same kind of way that 
the earth's surface has no beginning and no end, but returns 
upon itself without being infinite in extent ; and if this idea of the 
curvature of the space-time continuum is accepted it means 
that light from the stars does not travel outwards in every 
direction, but curves round the universe and returns on itself. 
It means, for example, that if at midnight we pointed a tele- 
scope, sufficiently powerful, to a point in the sky opposite the 
sun, we should see it as a faint star shining with the light it 
emitted 500,000 million years ago, this being the time calculated 
for light to travel once round the universe (Hubble). 

The universe is so vast that figures denoting dimensions 
convey little to the imagination, and even when distances are 
expressed in such large units as light-years (p. 320), the figures 
are too big to really comprehend. Outside our own galactic 
system, which occupies an insignificant fraction of the whole 
of space, there are 2 million nebulae, visible in the great ioo-j 
inch telescope, spaced more or less evenly at an average distance^ 
apart of about 2 million light-years, and the most distant is 
about 140 million light-years away. When the new 200-inch 
telescope is ready in a few years, we should be able to penetrate 
twice as far into space and see 2 3 , i.e. eight, times as many 
nebulae if they are there, which would bring the total to 16 
million. But it may be that, instead of opening up new fields 
as we explore farther and farther, we should simply see the 
nebulae which are nearest to us over again namely, by the 
light they emitted 500,000 million years ago, and which had 
travelled by the roundabout way instead of direct to us. 

It has, in fact, been seriously suggested that the back view 
of the Great Andromeda Nebula, as it was this long time ago, 
is now visible to us in the form of the faint nebula h 3433 at 
the opposite side of the sky, that the latter is the same nebula seen 
by light which has travelled the long way round the universe. 


This startling idea must, of course, not be taken seriously till 
there is more evidence, but there is certainly something strange 
in the behaviour of very distant nebulae. In order to make this 
more intelligible, let us adopt Jeans' model of the known 
universe based upon proportionately reducing everything in it 
to a size that we can comprehend. 

Imagine, then, the sun as a scarcely visible speck of dust, 
and the earth *s orbit around it (about 190 million miles diameter) 
the size of a pinhead; our nearest neighbour (Proxima Centauri) 
is then 225 yards away and the whole Galaxy about the size of 
the American Continent. Now let us try and explore extra- 
galactic space. We must travel 30,000 miles to reach a nebula, 
and then in all directions we find nebulae, about 30,000 miles 
apart, till we get a total of 2 million of them. Our model 
universe has now expanded so much that, in it, you can go about 
4 million miles in every direction before you reach the limits 
accessible to the loo-inch telescope; and each of the 2 million 
nebulae represents an " island universe " of thousands of millions 
of stars, either formed or potential as imperfectly segregated 
gaseous matter. 

Yet space is so empty of matter that even in its most con- 
gested regions in the centre of the Galaxy there are not more 
than six specks of dust for a volume as large as Waterloo 
Station (as Jeans puts it), whilst over the entire model the 
average would be more like one speck of dust for every 80 miles. 
Now returning to the distant nebulae in this model, the strange 
behaviour referred to above is this : they all seem to be scattering 
away from us in a most unaccountable way, at speeds which are 
excessive compared to the speeds of most other astronomical 
bodies. The velocities of recession are something like 1,000 
miles a second, and in one case even 2,350 miles a second. But 
perhaps it is well to say they seem to be moving away, because 
the only means we have of determining their motion and direc- 
tion is the Doppler effect (p. 323), which is the displacement 
(in this case to the red) of the spectral lines. 

Now de Sitter holds that the effect is apparent, and not 
real; he has, in fact, formulated a system of cosmology which 
differs from that of Einstein's relativity in one important 


particular. Whilst Einstein's relativity considers the space- 
time continuum as independent of matter and postulates a 
definite distortion of this continuum by matter (so causing 
the effects of gravitation, bending of light, etc.), de Sitter 
supposes that the size of the Cosmos is determined by the 
amount of matter in it, and that space has adjusted itself to a 
particular size, suited to the amount of matter contained in it. 
Moreover, Einstein supposes space and time to be mathematic- 
ally inseparable for anyone dealing with a limited fraction of 
the universe, but to become distinct for anyone who has the 
whole of space at his disposal, whereas de Sitter maintains 
" an equal partnership of space and time for the whole Cosmos/' 
And on this theory, into which we cannot enter here, he holds 
(to quote Jeans) that " the stream of time rolls more rapidly 
just where we happen to be than anywhere else "; and so he 
is able to predict that the displacements, to the red of the 
spectral lines, are mere distance-effects and not true motion- 
effects at all. 

This enables us to calculate the radius of the universe, 
which comes out at 2,000 million light-years, if the spectral 
displacements are proportional to distance. But they are not 
so proportional ; in fact, there is a good deal of irregularity, and 
some of the nearer nebulae are approaching us at about 200 
miles a second. The fact seems to be that the distance-effect 
of de Sitter is superimposed on real intrinsic velocities and the 
two effects cannot be disentangled; but if certain assumptions 
are made of a statistical type, concerning random velocities 
it is possible to calculate on the basis of both effects, and then 
the radius of the universe comes out at only 80 million light- 

It is impossible at present to know how much credence 
to attach to this last figure, but if it were true it would only 
be about half the distance of the farthest nebula which has 
been observed. In that case we are brought back to the idea 
that perhaps already we have penetrated beyond the confines 
of the universe, and are looking at what appear to be more 
distant objects, by a back view of the light from nearer objects, 
such light having travelled the long way round. According to 


de Sitter, if there were no matter at all, light would take an infinite 
time to go round, but in the presence of matter its speed has a 
finite value; also his theory demands a non-constant velocity 
of matter itself, mere motion, in fact, changing its speed in 
a manner which ultimately depends on the radius of the 

Altogether the problem is too baffling to find any solution 
as yet, or even comprehension. We have various estimates, 
by various investigators, of the size of the universe, lying 
between the extremes of a radius of 80 million light-years 
(de Sitter) and 84,000 million (Hubble), and between these 
extremes it is at present impossible to decide. 

In any case it is probably meaningless to imagine space and 
time by themselves, apart from matter, which occupies the 
continuum, however sparsely it may be distributed. Anyway, 
physical science is not only tending to this view, but is giving 
up trying to ''explain" the multitudinous phenomena of 
Nature; physicists are more and more getting away from the 
old idea that an explanatory cause must be found for everything, 
and more and more resting content with describing phenomena 
in terms of mathematical expressions which fit the facts more 
or less faultlessly. The tendency now, for instance, is to 
leave ether out of their explanations, simply because they 
can get on better without it. But this does not necessarily 
mean that it is not there ; it more likely means that the ultimate 
realities of Nature are never reached, because they may be 
beyond comprehension by the human intellect. 


Regarding the earth from the broadest possible point of view, 
we cannot fail to note that its materials are arranged in three 
distinct layers air, water and rock, sometimes called the atmo- 
sphere, hydrosphere and lithosphere. As we all know, the water, 
whether it be fresh or salt, is by no means uniformly distri- 
buted, but occurs in areas of every possible extent, from great 
oceans like the Pacific to mountain tarns. Next comes the 
land, similarly broken up into continents and islands, but 


differing vastly from the water in being very heterogeneous. 
The question arises, What lies below the land areas and the 
oceans ? If we take the density of water as unity, then experi- 
ment has shown that the average density of the rock-forming 
materials is about 27. Is the underlying substance denser; 
still ? We should expect it to be so, and we have abundant 
evidence to prove that our supposition is correct. 

The Composition of the Earth's Crust. An examination of 
the land surface to which we have access reveals to us that the 
crust is, in part, formed of stratified materials that have been 
laid down under water in far past ages, and of unstratified 
rocks that have had an entirely different (igneous) origin, 
and which may be collectively called granitic and basaltic. 
Granitic rocks (for which Suess proposed the generic name Sial) 
have a density of about 2*^wEilst basaltic rocks (Sima) reach an 
average of about 3*0. Over considerable areas of the land 
surface this heavier rock (basalt) is spread, and there is evi- 
dence to show that, in ages long past, and over and over again, 
it must have welled up (molten) to the surface and spread out in 
great sheets, through cracks in the crust from reservoirs below. 
Once exposed it was, of course, subject to denudation, and much 
of it, therefore, has been washed away, but what is left covers 
hundreds of thousands of square miles. There is also good 
reason to believe that the bed of the ocean is largely formed 
of basaltic rock, for oceanic islands, such as the Azores, that 
have never been connected with continents, are in the main , 
composed of basalt. The fact that these basaltic rocks or lavas 
are very uniform in composition points to a common origin 
i.e., from what might be regarded as a deep-lying reservoir 
beneath the stratified layers. 

So much, in brief, for the earth's crust. What is beneath 
this crust ? Geologists consider that below the basalt there 
are heavier rocks consisting largely of iron magnesium silicate 
(peridotite or Femi of density 3-3) and, deeper, perhaps a thick 
layer of heavy oxides and sulphides (density over 5), sur- 
rounding the central core of nickel-iron (Nife) of density 8*2. 

The actual density of this metallic core, whose radius 
is about half that of the earth, is calculated to be 12, and this 


higher figure is due to the great pressure of the superincumbent 
layers. These views as to the constitution of the earth are 
mainly derived from a consideration of the mode and rate of 
propagation of earthquake waves (seismology). Very little 
is known of interior temperatures (see p. 352), which must 
be high, though not high enough to bring about a molten 
condition (except probably the iron core) owing to the high 
pressures prevailing. This internal heat is probably mainly 
due to the earth not having completely cooled down from its 
original state of incandescence, but, as we shall see (p. 356), 
it is partly derived from small quantities of radioactive 
elements present in the igneous crust. 

To summarise, we may say that this crust is about 20 
miles thick, solid, and consisting of say 6 miles of sial above 
and 14 miles of sima below; beneath this is the vast rigid mass 
of femi, oxides and sulphides, sometimes known as the Dunite 
shell, about 1,800 miles thick, leaving a core principally iron, 
which may be molten, whose radius would be about 2,000 

The Foundations of Modern Geology. Modern geology is 
not much more than fifty years old, and regards earth structure 
from a point of view that did not present itself to the geologists 
of last century. We are fortunate in possessing authoritative 
statements on the subject by a distinguished geologist, Professor 
Joly of Trinity College, Dublin, of which we shall avail our- 
selves fully in the following pages. In a recent letter to the 
author, Professor Joly wrote: "The main factors have been 
revolutions, or periods of mountain building, isostasy and 
radioactivity. All modern geology rests on these founda- 
tions/' We cannot, therefore, do better than attempt, however 
imperfectly, to understand what these " foundations " are. 

There are one or two general features in the configuration of 
the earth's surface that must be noted before passing to details. 
These features may be readily appreciated with the aid of a 
good physical atlas, but Fig. 106 will give us the essentials. 

The first point to note is that the land areas are orientated 
north and south, being separated by the great oceans which 
stretch from pole to pole, both of which latter are themselves 



seagirt. Secondly, the general trend of the mountain ranges is 
also north to south, save in the case of the Eurasian mountains, 
where it is east to west. Thirdly, "the greatest mountains 
confront the widest oceans/' and run more or less parallel 
with the coast-line, even the (geologically speaking) recently 
formed Himalayas facing the Indian Ocean. The general con- 
clusion is that there must be an intimate connection between 
orogenesis, or mountain-building, and the ocean expanses, that, 

16 14 

14 16 18 


in short, " most of the great mountain ranges of the globe have 
been formed by thrusts from the nearest ocean basin " 

Another prominent feature of the earth's surface is the 
existence of cracks or rifts stretching north and south, such as 
the African Rift Valley (Fig. 106, dotted line), occupied now by 
the double chain of the great lakes, and, forking in the north, 
eastwardly as the Gulf of Aden and westwards as the Red Sea, 
continued as the Valley of the Jordan. Similar rifts on a 
smaller scale occur in other quarters of the globe, and all are 
due to tensional forces at right angles to their general trends. 

" He who has visited any of the great mountain regions of 


the earth is impressed by the greatness of the forces which must 
have prevailed. He judges the greatness of the forces by that 
' strength of the hills ' which they have overcome. He will 
also reflect upon the operation of other forces forces of the 
feeblest description slow and silent in operation. And here he 
comes to see that not less influential than those overwhelming 
forces which have uplifted the mountains is the time-element 
which integrates the feeble forces of friction and solution over 
geological areas. The one uplifts; the other pulls down. In 
the history of the earth, mountain ranges have come and gone; 
come and gone many times. The great forces uplifting; the 
feeble ones, aided by inexhaustible time, pulling down." 

" But there is obviously a mystery underlying the whole 
matter. Whence come the great constructive forces which to- 
day seem to be as great as they were in the most remote past ? 
They have not grown weary at their work; for the existing 
mountain ranges of recent date as we shall see are equal to, 
if they do not exceed, those of former ages." 

" The mystery deepens when we are told that these great 
folding forces proceed from the ocean, and that their magni- 
tudes are measured by the ocean-span." 

" We find also that crushing is not the only form of stress 
which has racked the continents. Irresistible tensions have 
also prevailed in the past, and we have presented to us the 
spectacle of one of the greatest of the continents rent from 
end to end by these tensile forces. We ask in turn whence 
come these tensional forces ?" 

" But these facts do not exhaust the element of mystery 
prevailing over the surface features of the earth. Our cities 
are raised on rocks which formerly were far beneath the surface 
of the ocean. Right across the great continents stretch the 
floors of ancient seas. And geological research tells us that 
not for the first time have these continental regions risen from 
the ocean to receive the light of the sun. There were repeated 
submergences and repeated resurrections." 

" Finally, and most remarkable of all, an orderly sequence 
has prevailed in these great events; the entire surface-history 
of the earth being, as it were, laid out according to a succession 


of these events, sundered by enormous intervals of geological 
time. The phenomena of the resurrection of the land and of 
the mountains are physically connected spring from a common 
source and in due course both land and mountain range find 
a common grave in the ocean waters rising over the con- 
tinents " (Joly, " Surface History of the Earth "). 

We have become so accustomed in our brief span of life to 
look on the majesty of the " eternal hills " and to ignore the 
erosion of their flanks and summits that has gone on unceasingly 
for countless centuries; to think of the oceans as vast quiescent 
expanses of water, ruffled, it may be, from day to day, by local 
storms, and to neglect the great forces ever active beneath 
their beds and under the land that rears its shoulders above 
the waters, that reflections such as those we have quoted must 
come to us almost as a revelation. Geologists in the past merely 
scratched the surface of the earth; geologists of today seek 
to know the greater things that underlie these surface scratch- 
ings, and to probe the mysterious forces that made the earth 
what it is. 

The Estimation of the Density of the Earth, As long ago 
as 1774 Maskelyne, the Astronomer Royal of those days, made 
the first attempt at determining the density of the earth. He 
based his work on a statement made by Newton that a plumb- 
line suspended near a mountain, whose weight and size were 
known, would be attracted to the mountain, and that, if the 
amount of that attraction were compared with the attraction 
of the earth as a whole, the density of the globe could be de- 
duced. Newton made the remarkably accurate guess that 
that density was about five and a half times that of water. 

Maskelyne made his experiment on an isolated, conical hill 
in Perthshire, called Schehallion, which reaches a height of 
3,547 feet. The volume of the mountain having been determined 
by survey and its density computed from that of its constituent 
rocks, the deviation of the plumbline on opposite sides of the 
mountain was calculated and found to be EFD (Fig. 107), which 
gave the difference between the pull of the whole earth and 
the pull of Schehallion. These experiments were repeated by 
Hutton and also by others on Arthur's Seat near Edinburgh, 


and on the great extinct volcano in Ecuador, Chimborazo, which 
is 20,498 feet high. The result of all these measurements was 
to show that the globe as a whole was about five times as dense 
as water. Since, as we have seen, the surface rocks vary in 
density from 27 to 3, it followed that the materials forming the 
mass of the earth must be much denser than these (see p. 343). 
Another method of tackling the question was that adopted 
in 1854 by Sir George Airy viz., by means of the pendulum 
but estimates made in this way were found to be unreliable. 

EV ID D'/ /E' 



Finally, a more accurate method was employed, based on that 
used by Cavendish in the end of the eighteenth century. 
Cavendish's apparatus was vastly improved by Professor 
Vernon Boys of Oxford, and used by him during the years 
1895-1900. The general plan of the apparatus may be under- 
stood from Fig. 108. Two small gold balls, b and &', each 
% inch in diameter, are suspended by very fine quartz fibres from 
the end of a rod, R, which carries a small mirror. R is in turn 
suspended by a quartz fibre from the pin P. B and B' are two 
lead balls, each 4^ inches in diameter, and capable of being 


twisted round so that B may hang behind ft, and B' in front oi 
6', B and B' attract 6 and ft', and the twist or torsion of the 
fibre carrying the rod R can be measured by the reflection from 
the mirror on a graduated scale. The whole apparatus is 
enclosed in an airtight chamber, and vibration eliminated 
as far as possible by conducting the experiments in the crypt 
of an Oxford college. It was found that the pull of the lead 
balls on the gold ones, against the earth's attraction, could be 
estimated even when it amounted only to one-millionth of a 
gramme. From numerous observations it was calculated that 
the density of the earth was 5*5268 times that of water, so that 
Newton's guess was almost exactly correct. 

Periodic Movements of the Earth's Crust. It is rather 
startling to be told that the land level is in almost constant 
movement, not merely movements of a seismic nature, of which 
we immediately think when the statement is made. When a 
high tide climbs the shore millions on millions of tons of water 
are added to the weight borne by the coast-line, and it is 
estimated that the West Coast of Ireland sinks about 3 inches 
at every high tide, to rise again at ebb, when the superin- 
cumbent load of water is withdrawn. When I inch of rain 
falls on a square mile of continental area, it adds a weight 
equal to 60,000 tons, but although this will induce a sinking 
of the land, the depression is not permanent, for the original 
level is restored when the water evaporates or drains away. 
But if a river deposits silt or mud the* deposit is permanent, 
and the gradually increasing weight causes a lowering of the 
land level, balanced, however, by the continual deposition of i 
more material Hence it is quite possible for stratified rocks 
to be as much as 3,000 or 4,000 feet thick, and yet to have 
been laid down in comparatively shallow water, the depression 
of the land keeping pace with the added layers of sediment. 
Conversely, continual denudation, scraping and washing off 
the land surface, therefore lessening the weight borne by 
the crust, may lead to a slow elevation, so that the surface 
remains at an approximately constant height. The different 
parts of the earth are thus to be regarded as in a state of 
isostatic equilibrium. 


Isostasy. The term "isostasy/' as applied to the earth's 
crust, may be briefly explained by saying that if we imagine 
a gigantic cheese-borer capable of scooping out a pillar of solid 
material from the surface inwards to a common distance from 
the centre, the pillars, if of the same diameter, would be 
approximately of the same weight, but not of the same height, 
for the continents stand higher than the beds of the ocean, and 
the materials of which they are composed are of lower density. 

This question made itself rather prominent during a sur- 
vey of India taken about the middle of last century. The 
survey and the plumbline did not give the same results at 
certain stations south of the Himalayas, and it was thought 
that the attraction of the great ''massif" to the north was 
the cause of the discrepancy. The geologist, Pratt, calculated 
what this attraction should amount to, assuming the density of 
the mountains as 275, and found it to be three times as much 
as it actually was observed to be. Why did the mountains not 
pull the plumbline more than they did ? Sir George Airy put 
forward a suggestion which has been universally accepted 
viz., that just as an iceberg standing 300 feet above water-level 
may have a depth of 2,400 feet below water, so a lofty range 
of mountains sends down a considerable portion of itself into 
the substratum. The mountains, owing to their great height, 
certainly do exert a plumbline attraction above, but this will 
tend to be equalised by a diminished attraction below, owing to 
the substitution of lighter materials for the heavier lava. The 
theory of isostasy " involves the view that the lighter continental 
crust floats upon a universal substratum of heavier materials/' 
Indeed, the parallel with the iceberg is fully upheld, for it 
has been found that " the emergent volume bears to the sub- 
merged volume approximately the ratio of i: 8," which, as it 
happens, is that usually shown by a floating iceberg, 300 : 2,400, 
as in the example just given. 

About 1920, Wegener put forward the hypothesis that the 
continents themselves are not fixed, but in a condition of slow 
drift, as masses of the sial, floating upon a hypothetical magma 
beneath, become detached owing to tidal or other strain. 
In this way, for instance, the American continent in past 


ages has sloWly drifted apart from Europe and Africa in a 
westward dire'ction, and this view is certainly confirmed by 
fossil similarities m the rock strata on each side of the Atlantic, 
and by some correspondence in the shape of the shore lines, 
As yet, however, it is impossible to test the truth of this in- 
genious hypothesis, and meantime it must be admitted that 
seismological evidence indicates that the rocks beneath the 
crust are very rigid. 

Those who have made a life-study of earthquakes tell us that 
the movements are in the form of waves, faster ones, termed 
" preliminary tremors," gradually becoming longer and slower; 
and they also tell us that the separation of these two kinds of 
waves can take place only in a homogeneous medium, for a 
heterogeneous medium would result in a regular jumble of 
waves. Working out the matter theoretically, Knott showed 
that these waves must travel below the continental masses, 
and he estimated that the homogeneous rocks through which 
they pass extend about twenty miles below the surface; and 
this estimate is confirmed by other evidence, which we need 
not consider. Further, we may note that the seismic waves 
travel along the ocean bed in general more rapidly than below 
a continent; a point in support of the view that the floor of the 
ocean is largely basaltic in its nature. The Sima basis even if 
plastic behaves as a rigid, elastic, highly heated solid (some- 
times known as the Diorite or Tachylite layer) and, below 
this layer, the vastly thicker Dunite shell (p. 344) is also solid. 

Radioactivity of the Rocks. All the rocks composing the 
earth's crust, including the extruded basalts, contain traces 
of radioactive elements that are continually changing into 
elements of lower atomic weight and giving out heat in the 
process. The parent radioactive elements are uranium and 
thorium, and these, as they disintegrate, degenerate finally 
into the common metal lead. The disintegration is ex- 
tremely slow, for it has been estimated that half of the 
uranium now upon the earth will have disappeared in about 
5,000 million years, and one-half of the thorium in about 
13,000 million years ! Now since the oldest sedimentary rocks 
were deposited in the primeval oceans not less but probably 


ar more than 200 million years ago, "geological time is but 
i small fraction of the period it will take to reduce sensibly the 
nfluence of radioactivity/' Potassium is ^also radioactive, 
liough very feebly so, but as it occurs in considerable amounts 
n many rock-forming minerals it must, as a source of heat, 
;>e taken into account (Holmes and Lawson). It is quite im- 
possible for us to go into the calculations that have been made 
is to the amount of heat that escapes to the surface of the 
>arth from its interior; all that we can do is to give the result 
viz., that the average temperature at the base of the con- 
tinents is about 960 C.4.e., rather below the melting-point 
of basalt. The deduction from the data that have been 
accumulated is that the substratum must be storing its own 
radioactive heat. "It cannot be passing upwards to the 
terrestrial surface, for the continental radioactive heat accounts 
for almost all that is escaping at the surface/' What becomes 
of it we shall see presently (p. 356). 

What is the condition of affairs under the oceans ? The 
ocean bed is, as we have seen, in all probability for the greater 
part basaltic in its nature, and any radioactive heat developed 
in its surface layers must pass by conductivity into the 
overlying waters; but in the deeper layers a level must be 
reached where conservation of heat must take place, and 
whence it cannot escape, since the upper crust will act in 
the same way as the continental rocks do over the substratum 
on land. It has been estimated that this conservation of heat 
occurs at a depth of about thirty miles. What the radioactive 
conditions are in the layers below we have no means of knowing ; 
but there appears to be a diminution in the amount of radio- 
active heat in the very deep layers. 

The Age of the Earth. The age of the earth has always 
been a fascinating question. The Chaldean astronomers 
thought the earth was born about two million years ago, but 
most of the ancient cqsmogonists made the genesis of the 
earth coincident with that of man. The Irish Archbishop Usher, 
in the middle of the seventeenth century, held that the creation 
of the world took place 4,004 years B.C., a fanciful date recorded 
in some Bibles at the present day. At the other extreme we 


have the views of the Brahmins who held that the earth was 
eternal. After the birth of geology as a science at the end of 
the eighteenth century, various attempts were made to solve 
the problem, more especially by estimating the time that it 
must have taken to deposit the vast series of stratified rocks, 
calculated to be at least 500,000 feet thick when piled one on 
the top of the other. The evolution of organic life also 
demanded a prodigious number of years, and geologists and 
biologists alike were naturally at loggerheads with physicists, 
like Lord Kelvin, who said that not more than 40 million years 
had elapsed since the earth was a molten globe. The discovery 
of radioactive bodies in the beginning of this century showed 
that Kelvin's estimate was hopelessly wrong. 

Another method of estimating the age of the earth was 
proposed by Joly in 1899, based on calculations of the amount 
of sodium in the oceans and the amount added per annum. 
After many observations made in all parts of the world the 
figures worked out to 12,600 million million tons of sodium 
in the oceans as against 156 million tons added year by year. 
This would give about 81 million years as the age of the ocean, 
but many authorities regard the figure representing the annual 
increment as far too high, and consider that 35 million is nearer 
the truth. If that be so, we should get 330 million years as the 
age of the oceans; but even that number is considered by Pro- 
fessor Gregory as far too small. In 1921 he told the British Asso- 
ciation that ' ' multiplication by five would not be excessive/' 

During the past few years, as we have said, an entirely new 
method of estimating the age of the earth has been put forward, 
based on the rate of disintegration of the radioactive elements, 
uranium and thorium. It is known that an atom of uranium 
generates ultimately eight atoms of helium and one atom of lead, 
while one of thorium gives six atoms of helium and one of lead. 
The atomic weight of uranium, which is greater than that of 
any other element known, is 238*1, and thorium has an atomic 
weight of 232*1; but both degenerate into forms of metallic 
lead which have not the same atomic weights as ordinary lead. 
Uranium-lead has an atomic weight of 206, thorium-lead 208, 
while that of ordinary lead, which is a mixture of the two,is"207'2. 



These varieties of lead are of course " isotopes" (p. 256), and 
though they possess the same chemical properties, they may yet 
be physically distinguished from each other. Now it has been 
found that a million grammes of uranium give rise to T ^Vrj- of a 
gram of lead per year, and the same amount of thorium to 
JTSTOTJ- f a gramme of lead in the same time. Lord Rayleigh, in 
1921, said that the rate of disintegration of uranium could be 
" relied upon to have been the same in the past as we now 
observe it to be." If we calculate the amount of lead present 
in any uranium-bearing mineral we have a datum on which 
to base an estimate of the length of time it has taken to pro- 
duce it. From such data Rutherford has calculated that the 
age of the earth is not less than 3,000 million years, and possibly 
may be rather more. Anthropologists and geologists agree 
in saying that man existed on the earth 300,000 years ago, 
so that humanity is now little more than in its babyhood. As 
Dr. (now Sir) J. A. Jeans puts it (Nature, March, 1928) : " He 
is still concerned with his cradle and his feeding-bottle, and is 
just beginning to look with questioning eyes on the universe into 
which he has been born/' Like the " three little maids " in the 
" Mikado," he is wondering " what on earth the world can be." 

Periodicity in the Revolutions. A general survey of the 
earth's surface reveals what, at first sight, appears nothing but 
a confused jumble of more or less stratified rocks, tilted upon 
end, twisted, contorted and folded over each other, layer after 
layer; and, underlying them, great masses of unstratified 
materials whose origin must have been entirely different from 
the stratified types, which had manifestly been laid down in 
the sea. It was this confused jumble that geologists of the 
nineteenth century, like Smith, Murchison and Sedgwick, set 
themselves to unravel, with such success that they were able 
at last to present us with an orderly sequence of the surface 
layers of the earth's crust, such as that given on p. 101. 

Later on, it became apparent that all the great mountain 
ranges Andes, Rockies, Himalayas and the lesser ones Alps, 
Caucasus, Carpathians, Urals were of comparatively recent 
origin, and carried on their flanks stratified rocks very much 
older than themselves. Moreover, it transpired that these 


mountain ranges had all been formed about tlie same time in 
a geological sense. Further, denudation had entirely changed 
the aspect of the mountains since they were first formed. The 
overlying strata had been stripped off and the roots exposed, 
revealing the existence in far past eons of yet earlier mountain 
ranges, whose worn-down stumps alone persist. The general 
conclusion forced upon us by the study of such facts is that the 
surface of the earth has passed through a succession of alternating 
phases of comparative quiescence and tremendous disturbance. 
First, the continents began to sink very, very slowly, and the 
ocean began to encroach on the land, pushing forward and 
retreating alternately, just as rippling wavelets creep up a 
low-lying shore, one wave reaching a level not attained by the 
next, but overpassed by a third. At length the high tide mark 
is reached and the reverse movements begin, retreat exceeding 
advance, and the land emerges once more. The next stage is the 
elevation of the mountain ranges. All the consolidated deposits, 
formed in the period when the seas covered the land, are heaved 
up by some huge subaquatic thrust from the ocean, acting 
approximately at right angles to the orientation of the uplifted 
ridges, crushing and folding them into all sorts of irregular 
shapes, and altering the levels they exhibited when they 
were laid down horizontally in the sea. So came an uplift 
from below, hoisting the ridges into mountain ranges, often 
thousands of feet above the level of the mass of the new con- 
tinents. Then followed a quiescent period while the forces of 
denudation carried out their silent labours, carving out valleys 
and sculpturing the peaks into the forms they ultimately 
assumed, until after, it might have been, millions of years, the 
continents slowly sank and the whole cycle of oceanic invasion, 
silt deposition, retreat and upheaval began once more. 

There is abundant evidence to show that there have been 
several of these mighty " revolutions, " as the American 
geologists call them, in the history of the earth, following each 
other at intervals of many millions of years; and the times of 
their occurrence have been determined in relation to the order 
of succession of the rocks as made out by the geologists of last 
century. The last great upheaval took place in tertiary times, 



when all the well-known mountain ranges that figure in our 
atlases were formed, and we are now living in the quiescent 
period following that upheaval, the period of slow sinkage of con- 
tinental levels, and witnessing the denudation of the lofty peaks 
and elevated plateaux that came into being long ages before 
man or even his anthropoid ancestors appeared on the earth. 

Causes of the Revolutions. Finally, we have to ask our- 
selves, what brought these revolutions about ? We have 

already seen that 
both the continental 
areas and the oceans 
rest on a basaltic 
substratum which is 
at present solid; that 
the continental rocks 
and those underlying 
them aref eebly radio- 
active; and that 
although heat is con- 
tinually being pro- 
duced it is not lost 
to any great extent, but is, on the contrary, accumulating. 
The substratum, it was pointed out, was nearing its melting 
temperature, viz. 960 C. as against 1,150 C. This difference 
and the latent heat of melting has yet to be supplied, and, 
from experimental data, Joly concludes that " about 33 
million years must elapse in order that the requisite heat may 
accumulate and fusion be brought about." When the solid 
substratum liquefies and the land support is therefore with- 
drawn, the continents must slowly sink relative to the ocean 
level, permitting the waters to spread over the lower reaches 
of the land. " The solid crust of the earth, consisting of the 
continents and ocean floor, is being stressed by the increasing 
volume of the substratum. The earth is, in fact, increasing in 
volume, and its solid crust is too small to fit a larger world. 
Two effects inevitably follow fluid pressure in the substratum 
and corresponding tensile stress in the crust." 

When the fusion (melting) becomes general the whole crust 







Y Alpine 


I Laramide 



f Permian 

v Appalachia 



Primary . 


I Caledonian 





| Laurentian 



of the earth is raised up, and when solidification follows and 
the interior contracts, the covering has to accommodate itself 
accordingly, and so the rocks become compressed, folded, and 
elevated into ridges, just as the skin of an apple wrinkles as the 
contents contract. 

The world has thus, during all geological time, been pulsating 
rhythmically in accordance with the accumulation and dissipa- 
tion of the heat of the basaltic basis, " fusion and expansion 
being followed in every cycle by consolidation and contraction " 
(Holmes) ; and this alternation of thermal conditions depends 
in turn on the radioactivity of the rocks. 

" We can best realise what that trace of radioactivity means 
to the life upon the earth by looking forward to a day when it 
will at length be worp out. Mountains, unrejuvenated, must 
then sink down into the plains. Continents worn away age 
after age by sea and sky must be washed irrevocably into the 
ocean. Air-breathing life upon the land and land vegetation 
must finally perish. For the earth itself will have ceased to 
breathe. And the mind of man, which alone comprehends it 
all, will have become part of the forgotten past " (Joly). 

But there are other factors of an astronomical kind, also 
to be reckoned with in considering the future of the earth. The 
small tidal influence which the sun is constantly exerting on the 
earth has the effect of making the latter, as well as the other 
planets, rotate more slowly, and recede from the sun; also the 
much greater tidal influence due to the moon will, according 
to Jeffreys, lengthen out the day, tiU after 50,000 million years 
it will be equal to forty-seven of our present days, and the moon 
will then have receded to its farthest distance from the earth 
before it begins to approach again (see p. 337). After a million 
million years the sun will radiate sensibly less heat than it 
does now, even if it has not before then passed over into an 
insignificant dwarf (see p. 311), so that from all these causes 
the mean temperature of the earth will drop by a matter of 
30 C. or so, sufficient to freeze the oceans and rule out the 
possibility of life as we know it now. Moreover, accidents 
may happen such as another wandering star coming so close 
as to deflect the earth's course round the sun, or even collide 


with it; or an asteroid might collide with the earth. Such 
things are remotely possible, but in the last degree likely, so 
that though the outlook is bleak, so far as the very distant 
future is concerned, it is probable that no great changes will 
occur in the next few millions of years. 

Ice-ages. The most likely changes in this relatively short 
period ahead are those slowly recurrent oscillations of mean 
temperature that have led to ice-ages in the past. We have 
already dealt with (pp. 103-105) the last great ice-age, but it is 
now recognised that earlier ice-ages have existed with inter- 
glacial periods in between, though it is not known how often 
the process has been repeated in the past. 

A simple explanation of these recurrent ice-ages has been 
given by G. C. Simpson, viz. polar shift and variation in solar 
radiation, to the extent of 20 per cent, or more, over periods 
of about 250,000 years between maxima. There is nothing 
improbable in the assumption that the sun is a slightly variable 
star of long period, for it is known to be so over the short period 
of eleven years associated with sun-spot activity. When 
the sun-spots are at a maximum the radiative energy of the 
sun is slightly greater than when they are at minimum, and 
this increased radiation falling on the earth causes a slight 
rise in the mean temperature. But there is a relatively greater 
increase at the equator than at the poles, and the result of this 
increased difference in temperature is (i) more activity in the 
general circulation of the atmosphere (seep. 368) and (2) greater 
evaporation from the equatorial oceans causing increased cloud 
and precipitation of rain. It is true that the climate of 
England is not appreciably affected by the eleven-year cycle 
because, being near the cyclonic track, the effects are swamped 
by other more or less accidental effects; but, speaking broadly, 
the rainfall of the world is at a maximum when sun-spots are 
at a maximum i.e., every eleven years. 

When we come to consider the greater amplitude of solar 
variation in the long periods of about 250,000 years, it is easy 
to see that there will be maxima in the precipitation of rain 
and snow, corresponding to the maxima of solar radiation. 
It is during the long time when the precipitation of snow is 


steadily increasing towards a maximum, in those regions where 
the mean temperature is about o C., that the ice-age sets in, 
for the snow is increasing constantly, while the solar radiation 
is as yet not powerful enough to melt it; and so the ice-caps 
surrounding the polar areas extend towards the equator, while 
those surrounding high mountains and tablelands extend to 
the lowlands. When later the sun reaches maximum activity, 
the increased heat now melts away all the accumulated glaciers, 
and the increased precipitation takes the form of rain; but 
a time eventually comes when the declining radiation once 
more reduces the temperature of these regions, causing snow to 
appear; and so another ice-age supervenes. 

If we now follow the course of events, the position becomes 
interesting. As the solar radiation goes on declining precipita- 
tion virtually ceases in the regions we are considering, there is 
less cloud and so more sunshine, which though it is weaker 
than when at maximum gradually melts away the snow; a 
long relatively calm and serene period follows, a dry inter- 
glacial period perhaps five times as long as the wet one 
mentioned above, during which the solar radiation is all the 
while steadily falling to a minimum, then later slowly rising 
again to a maximum, when the cycle of changes outlined 
above commences over again. 

On this ingenious theory of Simpson one complete cycle 
involves the following sequence : 

1. Ice-age with increasing solar radiation. 

2. Short wet interglacial period at maximum radiation. 

3. Ice-age with decreasing radiation. 

4. Long dry interglacial period, on each side of the point 

of minimum radiation. 

At the present time the world appears to be somewhere 
about the middle of period (4), and if so we may expect another 
glacial epoch to begin in about 80,000 years. 

The Earth's Atmosphere. A word should be said first 
about planetary atmospheres, as these help us the better to 
understand the case of the earth. Unusual interest attaches 
itself to the nature of "the atmospheres surrounding the planets, 


inasmuch as the possibility of extra-terrestrial life is involved, 
but it is surprising how little really is known. The telescope 
affords relatively little information, but the spectroscope is 
of value in revealing the fact that the light observed is merely 
that of the sun, reflected by the atmospheres or solid surfaces 
of the planets, with some special absorption lines, due to the 
atmospheres themselves. The planets therefore do not shine 
by their own light, and so must be relatively cool, on the surface 
at any rate. The radiometer (p. 321), moreover, has enabled 
the surface temperatures to be determined, and particularly 
the variation of temperature on Mars with respect to latitude, 
season, and the hour of day. These temperatures have already 
been referred to (p. 314), and from the observations in general 
useful conclusions have been drawn for example, that Mercury 
always faces the sun on the same side (like the moon with 
respect to the earth), and that this side has a scorching heat 
which would more than melt lead, while the other side is 
excessively cold; it has no atmosphere. The other extreme 
is found in the distant planets, Jupiter and Saturn, for example, 
showing a surface temperature of about - 173 C. These latter 
temperatures are very approximately those calculated for 
rapidly rotating black-body spheres, taking into account that 
the amount of radiation absorbed per unit area falls off as the 
inverse square of the distance from the sun calculations which 
show that Jupiter should have a surface temperature of- 152 
C. and Saturn -183 C. As these giant planets are by no 
means black, but have what is called a high " albedo " or 
reflecting power, which would involve loss of some 60 per cent, 
of the radiant energy, it may be concluded that the observed 
temperatures are somewhat in excess of what would be 
expected from the sun's heat alone that is, these planets are 
warm, and indeed may be very hot within. 

A theoretical method of approach, involving the kinetic 
theory (p . 224) , enables us to form better pictures of the probable 
facts than observational methods. It has already been shown 
(p. 337) "that in all probability the planets were born out of the 
incandescent gaseous matter forming the surface of the sun, 
matter containing practically all the elements, but some like 


hydrogen, oxygen, silicon, calcium, and iron in greatest abun- 
dance. When this gaseous matter settled down into globes, 
more or less liquefied by cooling, the still gaseous envelopes or 
atmospheres must have undergone considerable changes during 
the long process of cooling down. These changes can be easily 
visualised in the case of the earth, of which we know most, but 
the picture we are now going to draw (pp. 361-365) is of course 
essentially speculative. The heavier elements (principally 
iron) must have first settled to the centre, followed by calcium 
and silicon compounds, etc., floating as a sort of siliceous slag 
on top, with an incandescent, greatly swollen atmospheric en- 
velope consisting of the lighter elements. As the latter cooled 
in the outer regions combination of these elements, to form 
compounds, must have occurred, the heavier compounds even- 
tually gravitating to the liquid core beneath. Finally, a hot 
atmosphere consisting principally of steam and carbon dioxide 
must have remained, but probably much of the lighter portion 
of the hot atmosphere escaped into space during this long 
process of cooling. 

Primeval Earth's Atmosphere. It must be remembered 
that the mean velocity of molecules is greater the lighter they 
are and the higher the temperature (see p. 224) . At the earth's 
surface if there were any exceeding seven miles a second they 
would gradually escape ; but with an inflated atmosphere a pro- 
portion of these molecules would be so far from the earth's centre 
of gravity that the critical speed for escape would be even less 
than seven miles a, second, since the gravitational potential 
falls off with the square of the distance from the centre. It is 
probable that a good deal of hydrogen was lost in this way, 
and it is certain that if the earth had been less massive it would 
have lost a good deal more of its substance than it has done. 
The kinetic theory enables us to say what gases would be lost 
for any planet, given the mass of the planet, the temperature 
and distance (from the centre of gravity) of the molecules of 
the gas. The planet Mercury, for instance, is now too hot 
and too small to retain any atmosphere, and Mars has lost, 
in all probability, an enormous amount of its lighter materials, 
whilst the moon, although of much the same mean temperature 


now as the earth is, cannot retain any atmosphere because its 
gravitational potential is too small. 

There must have come a time in the history of the earth 
when the liquid or solid globe was still red-hot and the steam 
atmosphere weighed some two or three hundred times at least 
what the present atmosphere does. The original carbon dioxide 
had by now largely entered into combination with mineral 
components of the surface forming carbonates; there was, 
presumably, a small percentage of nitrogen in this hot atmo- 
sphere, though probably no oxygen, as this chemically active 
element would more likely have entered into combination with 
mineral components of the solidifying crust. High up above 
this steam atmosphere we may picture a vast canopy of clouds 
of enormous thickness, reflecting from their upper and outer 
surfaces the bright sunshine, none of which could penetrate 
through, and precipitating beneath deluges of condensed water 
with appalling discharges of lightning and thunder. This 
precipitated water could only exist within a limited upper region 
of temperature and pressure, and in the lower, hotter strata, 
above the critical temperature (365 C.) , the liquid would change 
to the gaseous condition. At the surface itself, owing to the 
pressure of say 200 atmospheres, the density of the steam 
must have approximated to that of water itself (say one- 
fifth), and at this temperature and concentration water is 
a very reactive chemical substance, almost acidic in its 
properties; so that enormous solution in, and hydrolytic 
decomposition of, the crystallising rocks beneath must have 

The above picture may represent what is happening on 
Jupiter at the present day; owing to its vast size it would 
presumably not have reached the stage of cooling which the 
earth has reached today, and it is certain from telescopic 
observations that the atmosphere of Jupiter is in a condition 
of most stupendous turmoil. The surface we see may actually 
be hundreds if not thousands of miles above the hot liquid 
or semi-solid core beneath, and the temperature (-173 C.) 
observed is not necessarily inconsistent with such a condition 
of things, since by adiabatic cooling (see p. 366) there would 


be an enormous difference between the temperature of the core 
and that of the upper atmosphere. The visible surface of 
Jupiter and Saturn, on this view, represents the tops of clouds 
(which may or may not be water) of enormous thickness; and 
the more rapid rotation of the equatorial regions as compared 
with those of higher latitude may be due, as in the case of the 
sun, to the shape of the core itself being equatorially elongated 
rather than spherical, owing to a speed of rotation very much 
greater than the observed ten hours or so for the surface of 
gaseous envelope. 

Returning to our imaginary history of the earth's atmosphere, 
a time came when the cooling was sufficient to bring the lower 
strata to the critical temperature, and when this happened the 
highly compressed steam beneath gradually contracted and gave 
birth to a boiling ocean whose temperature steadily fell from 
365 C. to its present-day value. How long the entire cooling 
process took we cannot even guess, but it must have been 
presumably many million years, being, in fact, retarded by the 
dense steam clouds, above, serving to slow down the loss of heat 
by radiation. During the later period the pressure of the 
atmosphere was steadily diminishing as more and more steam 
became water; and eventually practically nothing remained 
except the original amount of nitrogen, with a little carbon di- 
oxide and a minute proportion of the inert gases, all accompanied 
by a good deal of water vapour as moisture. The earth had now 
cooled down to say 50 C., a state which we may suppose is 
similar to Venus at the present day, the steamy atmosphere 
being surmounted by a thick canopy of clouds, through which 
the sunshine could only penetrate with difficulty. Venus, on 
this view, although it is practically the same size as the earth, 
has cooled down more slowly, because being nearer the sun it 
receives twice the amount of solar radiation (see p. 258) ; and 
the bright surface which we see consists of the cold tops of 
these clouds. The solid surface of the planet is never seen, 
but it is probably at a temperature of about 50 C., and 
spectroscopic observation has shown that oxygen is absent, or 
at any rate less than "I per cent, of the terrestrial amount in 
the region above the clouds. The reason for the difference in 


temperature between surface and upper atmosphere will be 
dealt with presently. 

Beginning of Life. Returning once more to earth, we may 
presume that, somewhere about the period we are speaking of, 
with a warm ocean, chemical activity of all kinds must have 
been much greater than it is now, for in general chemical velocity 
more than doubles itself for each rise of 10 C. ; and it is possible 
that, in the welter of chemical changes taking place, complex 
carbon compounds (organic substances) were produced, some of 
these of a colloid nature (see p. 407), functioning as activators 
of chemical change, or what we call catalysts (see p. 396), such 
as enzymes (p. 447) are. These catalysts would be the precursors 
of the lowest forms of life, which may have been similar to the 
so-called auto-trophic, bacteria viz., the nitrite- and nitrate- 
bacteria (see p. 419), whose life depends mainly on the chemical 
energy contained in inorganic compounds. 

Other low bacteria (unicellular organisms, p. 419) have similar 
functions; thus sulphur-bacteria convert sulphides to sulphates, 
iron-bacteria convert ferrous compounds to ferric, while 
nitrite- and nitrate-bacteria convert ammonia into nitrates. 
In all these cases carbon dioxide is utilised somehow in 
building up protoplasm, the energy being derived by oxidation 
of the sulphide, ferrous compound, or ammonia respectively. 
These bacteria, therefore, require oxygen; but there are others, 
called anaerobic bacteria, which do not require oxygen but 
derive their energy from the chemical changes which they 
induce, and it is possible that the first living organisms which 
appeared derived their energy, not from oxidation by oxygen 
but from nitrites and nitrates, which must have abounded in 
the primeval ocean owing to violent thunderstorms (which are 
known to produce nitrous and nitric acids). 

However this may be, certain it is that when the sun's 
rays were able to penetrate through the clouds a new group of 
complex organic compounds (chlorophyll group see p. 438) 
became associated with the low forms of life, and a new epoch 
was ushered in. For it is the function of chlorophyll, under the 
influence of solar rays, to decompose carbon dioxide, liberating 
oxygen and building up synthetically the materials of living 


protoplasm as we know it now. Oxygen now began to appear 
in the atmosphere and the new forms of life became to a large 
extent dependent on it, both plants and animals; and from 
this time onwards oxygen formed the most important consti- 
tuent of the atmosphere. Its amount today (21 per cent, by 
volume of dry air) is maintained by the plant life of the earth, 
whilst the carbon dioxide ("04 per cent.), which is a product 
of respiration of both plants and animals, is maintained at this 
level mainly through its continual assimilation by plants and 
its solution in the waters covering the earth. 

The other components of the atmosphere of today besides its 
principal one (nitrogen, 78 per cent, by volume of dry air) are 
hydrogen and the inert gases argon, helium, neon, krypton, and 
xenon (all in traces except argon, nearly i per cent.) . Moisture, of 
course, is very variable (i to 5 per cent.), dependent on humidity 
i.e., climate and weather. Of ozone (0 3 i.e., a molecule of | 
oxygen containing 3 atoms) there is none, except in the highest 
reaches of the stratosphere (p. 369), where probably also there 
is relatively more hydrogen and helium. These upper reaches 
are warm and partly ionised (see p. 255) by the intensity of 
solar radiation, and the ionisation is responsible for the Aurora 
Borealis, as well as the deflection of radio-waves by the so-called 
Heaviside layer, at about 30 miles height by day and about, 
60 miles at night. The pressure and density of the earth's 
atmosphere fall off rapidly with height, being only about a 
half of the surface value at about 17,000 feet (above the top 
of Mont Blanc) ; while at the average height of the troposphere 
(p. 369), or say 7 miles, the pressure is less than one-fourth of an 
atmosphere. But there is still a perceptible atmosphere at 
heights of hundreds of miles, and Jeans has calculated that 
even at 1,800 miles the density is such as to give 300,000 mole- 
cules to the cubic centimetre, but this is only one hundred 
million millionth of the number at the surface. 

Meteorology. The earth's atmosphere when dry and clear 
of clouds or smoke, etc., is very transparent to solar radiation, 
but the ozone of the upper regions cuts out radiation of the 
far ultra-violet below a wave-length of 2,885 Angstrom units 
(p. 259), and in any case 37 per cent, of the total incident solar 


radiation is lost in the upper regions by reflection and scattering 
(see p. 270). As it is the wave-lengths in the blue region of the 
visible solar spectrum which are most scattered, the sky appears 

The variable moisture content of the earth's atmosphere 
plays an enormous part in determining weather and climate. 
Even if there are no clouds this moisture, being much more 
opaque to infra-red radiation than oxygen and nitrogen, cuts 
off a lot of such radiation, but since it also cuts off relatively 
more (see p. 266) from the earth itself, moisture tends 
to conserve the earth's surface-heat by preventing excessive 
cooling at night. The formation of clouds is due entirely to 
adiabatic cooling of moist air by rising. It has already been 
pointed out under the kinetic theory (p. 226) that when air rises 
to a region of lower pressure and expands, it cools itself in so 
doing. The adiabatic laws controlling this important cooling 
process have been developed with great mathematical accuracy, 
and they enable us to calculate what the temperature fall will 
be for all conditions of temperature and pressure initially. 
Broadly speaking, the calculated fall, or so-called lapse-rate, is 
of the order of 10 C. per kilometre rise, near the surface for 
normal conditions. A kilometre is rather more than 3,000 feet, 
so if we could take up expanding dry air to this height and 
prevent any heat getting into it (from the sun or the earth by 
radiation) it would be 10 C. cooler than when we started. And 
if air could thus rise from the surface to ten times this height, 
say the top of the troposphere (about 7 miles for England), 
the temperature would drop nearly 100 C. Such a lapse-rate 
would not occur with moist air, because, as the moisture is 
precipitated out by the cooling, the latent heat of steam-con- 
densation (p. 236) to some extent compensates this adiabatic 
fall of temperature while cloud is being formed. 

As we have seen (p. 233), the vapour pressure of water is 
lower the lower the temperature, so that if the original moisture 
content which the air carried exceeds this reduced value, when 
the air is cooled to any temperature, the surplus moisture must 
come out as a cloud of liquid water viz., to the saturation 
point of the cooled air (dew point). The cooled air has now 


only that amount of water left in it which corresponds to the 
lower value for the vapour presure. In other words, a cloud will 
begin to form at that level, at which the water content (what- 
ever it happens to be) just exceeds the vapour pressure corre- 
sponding to the reduced temperature, for that level. But if the 
current of air goes on rising, still more moisture will separate 
out as the temperature continues to fall. The cloud, therefore, 
will go on increasing in height, but its base level will not alter 
so long as the moisture content of the air supply does not 

All sorts of clouds at different heights, up to about 7 miles 
in the latitudes of Europe, are formed in this way, and if the 
question is asked, Why does this rise of air take place ? the answer 
is to be found in solar radiation, either directly or indirectly. 
For example, the sun might so heat a patch of ground, which 
absorbs radiation more freely than the neighbouring region, 
that the air above it is heated and expanded to a point that its 
density is relatively reduced; it would then rise like a balloon, 
though in much less regular fashion, the denser air constantly 
pushing in beneath and displacing it, with much turbulent and 
rolling effect due to friction. This effect of any fluid rising, 
because its density is lower than that of its surroundings, is the 
well-known phenomenon of convection, and, of course, it is a 
very familiar effect everywhere. The " balloon " of air will go 
on rising (and partly mixing with neighbouring air) till its 
buoyancy is lost, for as it self-cools by expansion, there must 
come a time when the density difference vanishes as compared 
to its surroundings at the higher level; the cloud then ceases to 
rise. If the initial density-difference is very great, and the 
initial humidity considerable, the air will rise to much greater 
heights than usual, especially if the upward rush is maintained 
by a moist supply from below. 

Huge quantities of water may be thus precipitated out and 
come down in thunder-rain, accompanied by hail when the up- 
rush has caused a big drop in temperature in the upper regions. 
Whether any cloud produces rain or not will of course depend 
mainly on the amount of moisture in the rising air. So long, 
in fact, as the moisture particles are small their rate of fall is 


so small (see Stokes' law, p. 406) that they are carried upward, 
but when they increase, the rate of fall increases, and at certain 
size the rate of fall just balances that of the rising air through 
which they are falling and they remain stationary i.e., in the 
vertical sense; whilst any larger particles, whose rate of fall 
exceeds that of the rising air, will fall as rain. 

General Circulation of Air. Although, as explained, the 
sun may be directly responsible for rising air masses, more 
frequently the effect is indirect, by acting through the general 
circulation of the earth's atmosphere. Considering the earth 
as a whole with the sun's rays impinging upon its spherical 
surface, it is easy to see that the absorption by rocks, plants, 
water, etc., will be greatest in equatorial regions and least in 
polar regions, because of the average angle at which these rays 
strike the ground or sea the nearer this angle is to a right 
angle the more gain there will be by absorption and the less 
loss by reflection. So that in general the equatorial regions 
will be hotter than the polar; and, of course, this is true of 
other planets, like Mars, whose axis of rotation is more or less 
vertical to the plane of the ecliptic. 

But this is not all the story. When the sun's rays strike 
the earth obliquely as they do in temperate zones, and as they 
always do near sunrise or sunset, the amount of radiation 
reaching ground level is greatly reduced by another factor 
namely, atmospheric absorption. It will be easily understood 
that in these cases the heat rays have to traverse a length of 
hundreds of miles of atmosphere, the journey being greater 
because it is curved and not a straight line, owing to the effect 
of refraction; and although dry air is wonderfully transparent, 
moist air is relatively opaque. Moisture, which is always 
present even in dry desert regions, and also dust, from which 
the air is never free even over the ocean reaches, cut out quite 
a considerable amount of solar radiation not only by absorp- 
tion but also by scattering. The net result of these effects 
should be that the upper air (stratosphere) should be colder and 
the lower air and earth's surface warmer at the equator, whilst 
over the regions of high latitude the reverse should hold. As 
a matter of fact, the stratosphere over the polar regions is 


50 C, warmer than that over the equator, though the surface 
temperature may as much colder. 

The relatively high temperature, found in the very outer- 
most confines of the atmosphere, seems to be greatest over 
equatorial regions, though this has not been yet explained. 
At any rate, it is now fairly clear that the stratosphere, the un- 
disturbed rarefied and serene atmosphere lying above the 
chumed-up lower regions below (troposphere), has not actually a 
constant temperature as was once supposed, but a temperature 
changing slowly with height, in the opposite direction to that 
which obtains with the troposphere. With the latter, because 
it is churned up more or less continually by convection (being 
heated mainly from the warmed earth below), the temperature 
falls with increase of height. It is true it does not fall to the 
full adiabatic extent of about 10 C. per kilometre rise; this is 
because the churning is incomplete and because heat-intake 
is not excluded, since the troposphere itself absorbs some of the 
heat rays of the sun and earth. In fact, the average lapse-rate 
is only something like 6 C. per kilometre at low levels and 
about 7-5 C. at high levels of the troposphere; and, as can be 
imagined, since the troposphere represents the churned portion 
of the atmosphere it reaches much greater heights at the 
equator (about 10 miles) than it does in the middle latitudes 
(about 7 miles) ; while at the poles it is much less. 

Now in the stratosphere, which is the upper portion of the 
atmosphere resting on this disturbed troposphere, the heat 
intake is not derived to any appreciable extent from below, 
but mainly by absorption and scattering of solar radiation 
above, and so the outside (uppermost) layers are the warmest. 
For example, at the equator the bottom of the stratosphere 
(top of troposphere) has a temperature of about 195 absolute 
(at about 10 miles height) ; but as the higher regions are explored 
the temperature rises steadily until at a height of 36 miles 
it is about 303 absolute (30 C.) according to Lindemann and 

The exploration of the upper air has been very helpful in 
tracing out the general circulation of the atmosphere over the 
globe, but even now this is only imperfectly understood, 



Neglecting the complication of local and surface circulations, 
there appears to be a general continuous drift of air in the 
troposphere from the west, in both North and South Hemi- 
spheres above latitude 30, whilst the direction is opposite in 
the equatorial belt. The first or circum-polar drift from west 
to east makes it appear as if an atmospheric shell were rotating 
in the same direction as the earth itself, but rather faster, so 
that to anyone on the solid ground it seems to be moving as a 
wind of say 20 miles an hour. But in addition to this movement 
from west to east there is a component of motion, equivalent 
to a gradual drift towards the two poles, probably to feed the 
so-called polar anticyclones, or regions of high pressure. These 
can be roughly pictured as a cap of cold air (in each case) 
sending out a reverse drift or supply of cold air from east to 
west, with a general tendency towards the equator. This polar 
easterly drift, however, is a surface one, while the westerly 
drift near the poles is an upper one, so that in north polar 
regions we have a tendency (apart from local considerations) 
to east and north-east winds below, with west and south- 
west winds above; each opposite drift being independent and 
the upper one supplying the general air-feed for the lower one. 
On the other hand, in middle latitudes the entire drift, down 
to the surface, is westerly (apart from local circulations), 
but there is a region between the polar and middle latitudes 
where these opposed drifts of air (the westerly relatively warm 
and moist, the easterly cold and dry) come into conflict. The 
northern Atlantic and the north-west of Europe, for instance, 
constitute such regions of conflict, as do the latitudes of 
the so-called "roaring forties" in the Southern Hemisphere; 
and such regions are particularly the birthplaces of storms 
and cyclones (see p, 373), 

It is not easy to account for all these movements in the 
world circulation, but they are evidently connected with tem- 
perature differences between equator and poles. As the surface 
air over the poles is cold, while the upper is relatively warm 
(p. 369), the air is more dense near the polar surf ace than the air 
over the equator. The effect of this, as can be shown mathe- 
natically by the adiabatic laws, is to cause atmospheric pressure 


over the cold regions to be more concentrated at lower levels 
than is the case in warm regions. That is to say, at a high level 
the pressure is higher over the warm regions than it is over the 
cold at the same level ; or, putting it more precisely, the pressure- 
lapse with height is smaller over the equator than it is over 
the poles. Here the air collects, as it were, in a dense pool 
at the surface. 

So it comes about that, at high levels over the earth's sur- 
face, there is a steady fall of pressure, for any particular level, 
towards the poles where it is lowest, though the total pressure 
(surface) at the poles is relatively high. Now air at high 
pressure always tries to move to any place where the pressure 
is low, and so there will be a tendency for the upper air of low 
latitudes to flow to the poles. It cannot do so directly because 
of the rotation of the earth which deflects it to the east. This 
will be appreciated when it is remembered that the velocity of 
rotation (as say miles per hour) of any object on the earth's 
surface is greatest at the equator (about r,ooo miles per hour), 
just one-half at latitude 60, and zero at the poles. So any object 
starting to move say due north from the equator, endowed 
with this extra horizontal component as it travels to more 
slowly-moving parts of the earth's surface, will soon be deflected 
in an easterly direction; and eventually if it gets far enough 
it will appear to be coming from the west. Conversely any 
object starting to move from the North Pole, will, in its southerly 
movement, soon be left behind (as it were) by the earth and 
appear to be coming from a north-easterly and finally easterly 

This is exactly what happens with the wind spreading out- 
wards from the cold, dense air at the poles, impelled towards 
southern mid-latitudes by higher pressure; such wind quickly 
acquires a direction towards the west, and the force of these 
easterly winds, mainly derived from the above effects of the 
earth's rotation, may be enormous. We shall see later that it 
is similarly the earth's rotation which makes the wind, spreading 
outwards from any local region of high pressure (anticyclones) 
move circularly around them, with only a small component 
leaking directly outwards; and the wind drifting inwards into 


any region of low pressure (cyclones) is similarly deflected, 
but in the opposite direction, so that it mainly flows circularly 
round instead of directly filling up the low pressure area. 

We are now in a position to understand how it comes about 
that masses of air are so frequently rising by displacement 
from below (apart from direct convection due to solar heating), 
and thereby producing cloud, with or without rain. For in 
the movements of air outlined above it frequently happens 
that cold, dense air, near warm, light, and moist air, undercuts 
it and lifts it bodily off the ground. This happens in England 
on the northern side of advancing cyclonic depressions, where 
there is"usually"a north-east or east wind at the surface and 
a south-west wind above, sliding over it and continually 
rising. This gives gloomy weather, with thick clouds and often 
steady rain. On the other hand, on the south side of the centre 
of the depression, the air is rising for another reason namely, 
because it has a lower pressure and density than the air further 
south ; as wind blowing continually from the south-west, the air 
is also gradually displaced upwards, while southern air drifts 
in. This usually gives cloud, intermittent rain, and drizzle. 
On the western side of the depression there may be a very 
sharp line of discontinuity, where a veritable wall of cold air of 
polar origin comes tumbling down, as it were, and undercutting 
the air of southern origin. This usually produces violent 
effects squalls, more or less severe, and heavy rain showers, 
the so-called "clearing showers" that precede the relatively 
fine weather in the rear of the depression, where the cold, clear 
air of polar origin comes in from the west or north-west (p. 375) . 
It will be the^fore realised that any conditions that lead 
to atmospheric instability or strong convection must cause bad 
weather. Normally the atmosphere is fairly stable because 
the temperature lapse-rate (p. 369) is well below the adiabatic, 
and any undue rise by convection is promptly checked by 
adiabatic cooling. But if for any reason the upper air is 
unusually cold or the surface air unusually hot, while the upper 
air is normally cold, so that the lapse-rate exceeds the adiabatic, 
instability results, with more or less violent convectional effects. 
When the upper air is warmer than usual, as it is with the 


central regions of anticyclones, conditions are stable, and only 
slight convection can occur. for the reason just given; in such 
cases it is possible to have surface air in summer heated by 
the warm ground, without appreciably rising to form cloud, 
while on a cold winter night the surface air may be cooled 
many degrees below the temperature of the air, a few hundred 
feet above. 

Weather Forecasting. In spite of the immense labours 
which have been devoted to the study of atmospheric circulation, 
the present-day position of meteorology is far from satisfactory, 
if long-date forecasting be taken as the criterion. Even the 
origin of cyclones and anticyclones, which more or less dominate 
the weather from day to day, is as yet imperfectly understood. 
A cyclone or depression was recognised in the nineteenth 
century as an area in which barometric pressure is below the 
average, the lines of equal pressure (isobars) being closed, in 
more or less circular form as seen in weather-maps, those of 
lowest pressure being in the centre. Such a system, which 
generally drifts (in high latitudes) eastward, is associated with 
more or less wind and cloud or rain, the wind blowing circularly 
in a counter-clockwise direction, but with an inward tendency 
near the earth's surface i.e., towards the centre. An anti- 
cyclone has an opposite structure of closed isobars, more or less 
circular, with the pressure highest in the middle; wind blows 
in a clockwise direction, with an outward tendency near the 
earth's surface. The weather associated with an anticyclone 
(which is often approximately stationary) is fine, especially in 
the middle, though there may be much cloud or even rain in 
the outer regions of the lower pressure. The above wind 
directions of cyclones and anticyclones refer to the Northern 
Hemisphere; in the Southern the directions in each case are 
reversed. Broadly speaking, we may say in the Northern 
Hemisphere that when the high pressure is to the south the 
wind is from the west, while when the high pressure is to the 
north the wind is from the east. The law of Buys Ballot 
states the case completely by saying that if you stand with 
your back to the wind low pressure is on your left. 

These two main types of atmospheric circulation appear all 


over the earth's surface, but they are now recognised as only 
incidents in the general circulation of the earth's atmosphere. 
There can be no question that cyclones in the middle or high 
latitudes, of both the Northern and Southern Hemispheres, 
are born in the conflict between the two opposing currents 
referred to on p. 370. The idea, which prevailed in the 
nineteenth century , % that cyclones were caused by an upnish 
of warm moist air has been abandoned in the twentieth 
century because this hypothesis is not in accord with 
observed facts. The modern conception is due to V. and J. 
Bjerknes, the Norwegian meteorologists (father and son), and 
thelTrosw, which matured during the Great War, is sometimes 
called the "polar front theory." Briefly it is that the birth 
of a cyclone, which for England is generally away out on the 
Atlantic, follows on the wave front separating the warm westerly 
(or equatorial) air, on the south side, from the easterly or polar 
air on the north side of the line of discontinuity. This line or 
rather surface of separation is a wavy one, and a protruding 
tongue of equatorial air, the tip of which proj ects northwards into 
the polar air, begins the birth process. This wedge or tongue 
has a "warm front" on its easterly side, and a "cold front" on its 
westerly that is to say, on the east side of the wedge the warm 
air is rising and sliding over the adjoining cold polar air, while 
on the other side the cold polar air (being more dense) is con- 
stantly slipping underneath the warm tongue. The rising air 
on the east side produces (by cooling) cloud and steady rain; 
the displaced air on the west side produces violent dislocation, 
with squalls, whilst in the middle of the tongue the westerly 
air may be rising and producing cloud or rain. It has been noted 
that cloud and rain are never produced except by the rising 
(and consequent cooling) of air carrying moisture, but it is not 
yet quite clear how the deficiency of air in the central regions, 
causing reduction of pressure, arises. During the above 
changes the cyclone definitely appears, with circular deflection 
of the air round the point of lowest pressure i.e., the point 
of the wedge. The cyclonic system follows the main drift 
eastwards, generally intensifying on its journey i.e., the 
pressure gradient becoming steeper and the wind accordingly 


stronger. So, on the west or rear side of the cyclonic depression 
a stream of cold polar air which had originally come from the 
east curls round and blows from the north-west or west ; and in 
the passage of such a cyclone over any spot, the change of 
temperature and wind direction, as well as force, is usually very 
marked when this polar air appears, thrusting its way in at 
the "cold front/' After a few days 1 travel the warm sector 
or wedge is lifted bodily from the ground by the polar air which 
has pushed in, and the wedge is said to be "occluded." At this 
stage the depression becomes more or less uniformly cold at 
all points, its drift to the east ceases, and the wind gradually 
moderates as the depression fills in and the pressure rises to 

It is to be noted that such depressions do not usually appear 
singly, but drift, like eddies in a stream, in a usually north-east 
direction across the British Isles. Between each advancing 
depression, which brings bad weather, there is usually a wedge 
of high pressure, causing temporary fine weather; and the 
average line of the track of such successive cyclones skirts 
northern Ireland and southern Scotland. Very frequently, 
moreover, to the south of the "main depression a smaller and 
often more violent disturbance, called a secondary depression, 
appears, similar in every way to, though smaller than, the 
main cyclone. 

Climate. There are some regions of the earth which are 
very stormy because they lie within the average track of the 
cyclones. Such a region is the North Atlantic stretching to 
Iceland and including North-West Europe, which is, therefore, 
subject to changeable weather. Other regions are favoured 
by more or less permanent anticyclones,such as the mid-Atlantic 
and Azores region. In winter there is one vast region of high 
pressure over Siberia, like a permanent anticyclone, due no 
doubt to the configuration of the land, which by radiation cools 
the superincumbent air and so increases its density. This huge 
pool of cold air is even colder (at the surface) and more steady 
than that of the North Pole, and it tends to flow outwards in 
all directions, but, of course, acquires a circular rotatory motion 
owing to the earth's rotation (p. 371), Offshoots of this cold 


anticyclone give Central Europe, Scandinavia, and even 
Southern Europe the severe winters they often experience; 
and even England may feel their effects. But it is in November, 
when the Siberian anticyclone is becoming stabilised, and in 
May, when it is breaking up, that its influence seems most 
regularly to affect the English climate by producing cold 

Indeed, the meteorology of England is so capricious that 
it is virtually impossible to say there is any regularity about 
it at all. There can be doubt of the existence of eleven-year rain 
cycles and the thirty-four year Bruckner cycle, so far as the 
weather broadly is concerned; but these and other cycles which 
have been established by definite " correlation data " are largely 
, swamped in England by what may be called chance effects, 
and it is these which make long-date forecasting impossible. 
Alexander Buchan undoubtedly recognised some annual 
periodicities, particularly the six " cold spells " about 
February 9, April 12, May n, June 15, August 9, and No- 
vember 9, as well as three warm spells ; but none of these come 
with unerring regularity, except the cold spells of May and 
November, which rarely fail. And although it is not known 
definitely why, these two occurrences are connected in some 
way with the surface redistribution of the air in the Northern 
Hemisphere, when the Siberian system, in either forming or 
breaking up, establishes Icelandic or Scandinavian offshoots, 
which bring cold wintry conditions to Britain. 

Such in very brief outline is the present-day position, but the 
realities are exceedingly complex. Forecasting nowadays is 
mainly based on a search for surfaces of discontinuity in the 
advancing depression, rather than on the old lines of predicting 
a sequence of changes likely to occur with its advance, dependent 
on assumed similarities of weather, for similar positions in the 
area of all depressions. Forecasting has undoubtedly improved 
with the new treatment, but owing to the kaleidoscopic changes 
continually occurring in local circulation, in a region like the 
Western Atlantic, the chance element must loom up very large. 



The composition of the earth, the nature of the individual 
" stuffs " out of which it is formed, and how these materials 
behave towards each other, were problems that exercised the 
brains of the alchemists from the days of Geber (p. 16), and 
although they had ever before their eyes two chief goals, the 
discovery of the " elixir vitae " and the transmutation of the 
baser metals into gold, they could not help finding new materials 
in the course of their experiments. Many of the elements 
e.g., iron, copper, silver, gold, mercury, lead and so on had 
been known from very early times, but others were gradually 
added to the list, so that, by the year 1800, some two dozen 
or more elements, in the sense we now understand the term, 
had been isolated from their compounds or found native. By 
1850, largely owing to the discovery of new methods of analysis, 
for which men like Berzelius and Davy were responsible, the 
list had risen to well over fifty, and by the end of the century 
textbooks on chemistry furnished the student with the names 
and characters of at least two dozen more. Some of these, like 
cerium, gadolinium, samarium, scandium, etc., were very rare 
and little more than museum curiosities; still they had their 
interest in being yet other types of " brick " out of which the 
world was made. In our own generation the catalogue has 
been still further extended, sometimes by the addition of half 
a dozen at a time like the inert gases or by the cluster of un- 
stable elements that have come to light in the study of radio- 

But the search for new elements has been less the goal of 
modern chemistry than the attempt to unify and co-ordinate the 
great mass of knowledge which steadily accumulated, viz. , by the 
search for fundamental laws and the formulation of theories to 
explain these laws. At the beginning of the nineteenth century 
a tremendous stimulus to further investigation was given by 
Dalton's atomic theory, incomplete as it was. The fundamental 
distinction between acids and bases was already recognised 
though imperfectly understood. The work of Black (p. 166) 
on lime and magnesia, grouped among the " alkaline earths," 


followed by that of Davy (p. 143) on caustic soda and potash, 
grouped among the alkalis, had made it clear that all these 
so-called " bases " were oxides or hydroxides of metals which 
could neutralise acids, forming salts. On the other hand, 
chemists were misled by Lavoisier into believing that acids 
owed their character to oxygen, and so for a long time missed 
the essential feature viz., hydrogen, replaceable by metals. 
It would be tedious and outside the scope of this book to trace 
out the slow and difficult steps, during the nineteenth century, 
or the successive theories which were discarded, before towards 
its close the real nature of acids, bases and salts was elucidated. 
Briefly the modern conception is based on the brilliant achieve- 
ments of Arrhenius and van't Hoff who established the theory 
of electrolytic dissociation (or ionisation) to explain the many 
puzzling facts which research had brought to light. On this 
theory an acid like hydrochloric acid, HC1, is partly split up 
in solution into electro-positive ions, H + , and electro-negative 
ions, QT , the former cations, the latter anions. When the 
hydrogen is replaced by a metal, say sodium, giving sodium 
chloride or common salt NaCl, the latter in solution is similarly 
split up into the cation, Na + , and anion, GT . This feature 
(electrolytic dissociation or ionisation) is common to all acids 
and salts; whilst bases, like caustic soda, NaOH, in solution are 
ionised into a metal ion, like Na + and the anion group, hydroxyl, 
OH"" which is common to all of them. In these ions the sign + 
means that the atom has lost an electron, and the sign ~ 
that the atom (or group) has gained an electron. 

The Classification of the Elements. It was only natural 
that chemists should attempt to arrange the elements in some 
sort of order, in the hope of finding family relationships between 
them. One type of classification was based on the occurrence 
of some striking feature, arbitrarily selected; another scheme on 
some equally prominent characteristic; but when compared the 
two schemes might give quite contradictory results. For in- 
stance, a classification into gases, liquids and solids broke down at 
once when it was discovered that a gas, like hydrogen, could be 
both liquefied and solidified, and that a liquid, like mercury, 
could be volatilised and frozen. Obviously the ideal classi- 


fication was that which took into account all characters, leading 
to a grouping of the elements where the members of each 
division had the largest number of points in common. 

One method was to divide the elements into metals and 
non-metals, but that proved unsatisfactory, inasmuch as no 
hard-and-fast line could be drawn between them. To take one 
example only: hydrogen, one of the commonest of the elements, 
is a colourless, tasteless, odourless gas, and, without doubt, 
is obviously a non-metal; but on closer examination it shows 
itself possessed of some of the characters of a metal, especially 
by its ionisability in acids, like metals in their salts. 

Another mode of arranging the elements is in accordance 
with their " valency/' or saturation powers in combining with 
each other. Since the years 1850-60 valency has been a subject 
much discussed among chemists, originally by men like Kolbe, 
KekuM and Frankland. It is fundamental and therefore 
important that we should gain some idea of the subject. The 
substances hydrochloric acid, water, ammonia and marsh gas 
are represented respectively by the formulae HC1, H 2 0, NH 3 
and CH 4 , which means that one atom respectively of chlorine, 
oxygen, nitrogen, and carbon combines with one, two, three, 
or four atoms of hydrogen; and seeing that no compound of 
hydrogen is known where the hydrogen is directly united to 
more than one atom of another element, it is spoken of as uni- 
valent, or monovalent. Chlorine in most compounds is similarly 
univalent. Oxygen unites with two atoms of hydrogen, and is 
therefore counted as divalent; nitrogen with three, and is tri- 
valent; carbon with four, and is quadrivalent, or tetravalent. 
Other elements have higher valencies up to seven or eight, 
while some, the "inert" gases, decline to combine with any 
other element, and are therefore said to be non-valent. 
Valency thus expresses saturation-capacity and may be stated 
conveniently as a figure which, for a given element, defines the 
number of atoms of hydrogen or chlorine which combine directly 
with one atom of the element. If oxygen, which is normally 
divalent, is taken as the standard instead of hydrogen or chlorine 
the valency figure is twice the number of oxygen atoms which 
combine directly with one atom of the element e.g., carbon 



A.N.=* Atomic number ; SY. Symbol ; D.D,=Date of discovery (approx.); 
A =a Ancient; A. W.=s Atomic weight when. O== 16. 


Name . 


















































































































































Niobium (Columbium) 




W/ V/J 




I8 44 



















































































,- 1926 









.- 1896 




























































* 1925 
















































(undiscovered) provision- 

ally Eka-iodine 


Niton (Radon) 





(undiscovered) provision- 
ally Eka-caesium 










* 1900 









- 1918 







N.B. This Table gives the list of elements as ordinarily found in Nature. Many of these 
elements are not '* pure/* but are mixtures of isotopes (p. 256) of different atomic mass (each 
practically an integer). The number of these isotopes among the radioactive elements (84-92) is 
considerable, and in these cases only the principal isotope is listed. 


is tetravalent in the compound CO 2 , calcium divalent in 
CaCl 2 or CaO, and so on. 

Some elements, however, are not constant in their valency. 
Thus phosphorus may combine with either three or five atoms 
of chlorine, and the same variability is true of many other 
elements. Frankland, more especially, studied this aspect of 
the question, and concluded that this inconstancy in valency 
depended on external conditions. As a result the classification 
founded on valency alone is unsatisfactory; further, it has been 
found that elements of the same valency may differ in almost 
every other respect. Contrast, for example, the opposite types 
of element sodium (univalent) and chlorine, which is generally 

The basis of the modern classification of the elements is 
atomic number (p. 249), plotted in periodic sequence, to be de- 
scribed presently. For purposes of reference the table given on 
pp. 380, 381 gives the names, symbols, atomic weights, atomic 
numbers and approximate dates of discovery of all the elements 
at present known. It will be noticed in this table that hydrogen 
is given an atomic weight, 1-008 and not i. The original reason 
for assigning this strange figure, instead of unity, to the simplest 
element was to make =16-00 exactly instead of a number 
slightly below this, as it is if H=i, and as oxygen appears so 
frequently in chemical compounds, this unit (exactly 16) made 
weight-calculations, based on chemical formulae, simpler. Later 
research work on the constitution of atoms has justified this pro- 
cedure, since as we have seen (p. 253) all elements which are 
"pure/ 'and all individual isotopes of "mixed "elements (p. 256), 
have atomic weights which are almost exactly whole numbers 
(multiples of i) if the atomic weight of the standard, hydrogen, 
is taken as 1-008. And this is evidently because, when Nature 
created the elements by condensation of hydrogen nuclei 
(protons) with electrons, the process was accompanied by the 
liberation of energy, which as we have seen (p. 296) means 
loss of mass energy and mass being interchangeable in Nature's 
magic crucible. Thus for example, when four hydrogen atoms 
weighing 4-032 are transposed into one atom of Helium, the 
weight of this atom is 4, the remaining mass being emitted as 


energy or lost radiation. How, when and where (whether at 
stellar temperatures or at the absolute zero of interstellar space) 
such genesis of the elements arises, is not yet clear; but it is 
certainly not yet within the power of man to achieve. It will 
be useful now to sketch very briefly how the modern classifica- 
tion of the elements has been arrived at. 

In 1808 Dalton (p. 184) propounded his " atomic theory/' 
which conceived matter as consisting of invisible and indivisible 
particles or " atoms/' those for each element having a definite 
weight, representing the combining weight. In the early years 
of the century every element was regarded as composed of in- 
dependent single atoms only, i.e., the smallest free units; for 
the term "molecule," though suggested by Avogadro in 1811, 
did not come into recognition until long afterwards. Avogadro 
said there were two kinds of primary particles, molecules 
integr antes i.e., molecules as we now understand the term 
and molecules elementaires, which corresponded to Dalton's 
atoms. That hydrogen could unite with oxygen was universally 
admitted, but that hydrogen could unite with hydrogen or 
oxygen with oxygen, to form pairs of atoms, respectively, 
as the real free units, was not realised until later as we 
have seen (p. 242). In 1858 the Italian chemist, Cannizzaro, 
wrote an important treatise on the whole subject, which he 
called " A Sketch of a Course of Chemical Philosophy/' in 
which he upheld the doctrine preached by Avogadro viz., 
that equal volumes of all gases, at the same temperature 
and pressure, contain equal numbers of molecules, " not," 
he said, " an equal number of atoms, since molecules of 
the different substances or of those of the same substance 
in its different states may contain a different number of 
atoms, whether of the same or of a different nature." In 
this way, through Cannizzaro's great genius, chemists at last 
stumbled on the truth and recognised that matter in its free state, 
as we commonly meet it, exists and reacts chemically or 
physically in the form of units or molecules, which normally 
consist of groups of atoms and not free atoms. That is, like 
compounds, the molecules of which are made up of two or more 
atoms of different elements linked together, elements consist of 


molecules made up of two or more of the same atoms linked 
together. Many of the elementary molecules consist (as gases) of 
groups of two atoms like H 2 , 2 , C1 2 , etc., some of three, like 3 
(ozone), but some more than three; whilst some notable cases 
occur, like mercury, Hg, of only one atom to the molecule, and all 
the inert gases (helium, argon, neon, krypton, etc.) are simi- 
larly mon-atomic. Cannizzaro also formulated a revised set of 
atomic weights corrected to the new basis, and showed that 
the laws of organic chemistry were identical with those of 
inorganic, although the contrary belief had for long been an 
obsession among chemists. " There is only one chemistry and 
one set of atomic weights/' was one of his favourite aphorisms. 

The Periodic Law. As far back as 1817 Dobereiner, 
professor of chemistry in Jena, found that many of the elements 
could be arranged in threes, or triads, the members of each 
triad having similar chemical characters and possessing atomic 
weights having a certain numerical relationship to each other. 
Thus chlorine, bromine and iodine formed a triad with like 
properties, while the atomic weight of bromine was the mean 
of those of chlorine and iodine or very nearly so. Similarly 
calcium, strontium and barium formed a triad with similar 
chemical characters, and the atomic weight of strontium was 
nearly midway between those of the other two. 

The next step was taken by the English chemist, Newlands, 
who, in 1864, discovered that when the elements were arranged 
in order of their atomic weights, and when any particular 
element was selected as a starting-point, the eighth element 
above it in the series possessed the same characters as the one 
from which the start had been made, like notes at octave 
intervals on a pianoforte keyboard. This, Newlands called the 
" Law of Octaves/ 1 but for various reasons chemists did not 
take kindly to this important discovery. Yet, five years 
afterwards, a similar "Periodic Law" of the elements was 
published by a much greater man, the famous Russian chemist, 

Mendeleeff was born in 1834 at Tobolsk, in Siberia, where 
his father was what we would call a Director of Education. In 
1856 he became a lecturer in the University of St. Petersburg, as 


it was then called, and two years later professor, a position he 
held until 1890, when he resigned to take up the duties of Director 
of the Bureau of Weights and Measures. He died in 1907. 

In his paper on the Periodic Law, published in 1869, h- 6 "^Hs 
us how he arrived at it. " There must be some bond of union," 
he says, " between mass and the chemical elements; and as the 
mass of a substance is ultimately expressed in the atom, a 
fundamental dependence should exist and be discoverable 
between the individual properties of the elements and their 
atomic weights. So I began to look about and write down the 
elements with their atomic weights and typical properties, 
analogous elements and like atomic weights on separate cards, 
and this soon convinced me that the properties of the elements 
are in periodic dependence on their atomic weights/ 1 As 
MendeLSeff '$ table is printed in every textbook on chemistry, 
it is unnecessary to repeat it here, more especially as it has 
undergone considerable modification of recent years, owing to 
our knowledge of the structure of the atom, acquired since his 
time. Without going into detail, the general principle may be 
illustrated in the following way. 

If we look at the table of the elements on pp. 380, 381, and 
put the names down in the order of their atomic numbers, begin- 
ning in the first row with the inert gas, helium, and the second 
row with the next inert element, neon, the third with argon, 
and so on, we arrive at the series begun below. 

In this series the figures stand for atomic numbers, but it is 
to be remembered that Mendel^eff based his sequence on atomic 
weights and not atomic numbers, which at that time were not 
recognised. This, however, makes practically no difference since 
the atomic number order is the same as that of atomic weights 
with few exceptions. It should also be noted that the inert 
gases of the first column of the (incomplete) series below i.e., 
He, Ne, A, etc. were not then known. But he clearly brought 
out the repetition or periodicity of similar properties, evinced 
by the elements in the vertical columns of his famous table. 

Firstrow He2 Us Be4 BS C6 N7 08 Fg 
Second row Ne 10 Na n Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 

Third row A 18 K 19 Ca 20, etc. 



It will be seen that in the first vertical row we get an 
association of inert elements; in the second column, Li, Na and 
K are closely allied in chemical properties, and similarly with the 
third and succeeding columns. As the table becomes more 
complicated further on, we need not pursue the subject; 
enough has been given to illustrate the principle underlying 
the law. 

In one interesting paragraph in Mendelfeff 's paper he says, 
referring to conspicuous gaps in his table due to undiscovered 
elements: t( The discovery of many yet unknown elements may 
be expected, for instance, elements analogous to aluminium and 
silicon, whose atomic weights would be between 65 and 75," 
and, sure enough, these elements which he had provisionally 
called eka-aluminium and eka-silicon, afterwards called gallium 
and germanium were discovered in 1875 and 1886 respectively, 
and their atomic weights came within Mendel6eff s predicted 
limits. His bold prediction of the properties of several other, 
as yet undiscovered, elements gave a great stimulus to chemical 
research; the predictions were verified as the new elements were 
discovered, and at the present moment there are only two gaps 
to be filled in (atomic numbers 85 and 87). Except these, no 
new elements remain to be discovered, though new isotopes 
may yet be found of known elements, and it is possible that 
elements higher than atomic number 92 (uranium) may yet be 
found. If ever found they would probably be very unstable 
radioactive elements, and Jeans as we have seen (p. 309) believes 
that they exist in great quantity in stellar material; so 
that the disruption of such hypothetical elements, with libera- 
tion of radiant energy, explains the great loss of mass which 
the sun and stars are believed to have suffered in the past 
(p. 330) and even the enormous stores of energy (mass) which 
they still radiate away into space. 

The recognition of the Periodic Law raised once more the 
question of the origin of the elements, and resuscitated the idea 
put forward by Prout in 1815 viz., that the atoms of all the 
elements were condensations of various numbers of hydrogen 
atoms. Stas's accurate atomic weight determinations, how- 
ever, negatived this idea, but, as we have seen (p. 283), all later 


research has gone to confirm the essential truth of Front's 

The Carbon Compounds. In spite of Cannizzaro's in- 
sistence on the fact that " there is only one chemistry/' it has 
been and still is the custom to divide the science into two 
main sections, Inorganic and Organic, the latter centring on the 
element carbon. For that reason organic chemistry is often 
called " The Chemistry of the Carbon Compounds/' Of course, 
the element carbon takes part in the formation of very many 
inorganic compounds also, such as calcium and potassium car- 
bonate; but numberless carbon compounds are associated with 
life, either directly or indirectly, life in the far past, as in the 
case of coal and mineral oils and their derivatives, or, in the 
present, as constituents of animals and plants, the proteins, 
carbohydrates and fats. Many of such bio-chemical com- 
pounds have been synthesised in the laboratory, beginning 
with the waste product of the animal body, urea, which was 
synthesised by Wohler in 1828. This historic synthesis is 
important, in that it disproved the old doctrine of " vital 
force," which laid down that organic compounds could not be 
produced chemically, but only by the intervention of life. 

It was well known to the chemists of the middle of the 
nineteenth century that carbon was a quadrivalent element, 
and could hold on, so to speak, to four atoms of a monovalent 
element such as hydrogen. Thus, the substance marsh gas, or 
methane, CH 4 , could be represented by the formula with four 




The carbon atom with its four saturation-valencies or four links 
was then said to be " saturated," as it could not be induced to 
take up any more hydrogen atoms than four. 

This method of expressing the formulae of compounds, as 
diagrammatic expression of the structure of their molecule, as 
was first introduced by Couper in 1858, when Kekul6 first 
recognised the tetravalency of carbon. The method, later 


applied to organic compounds in enormous numbers, is called 
the " linking of atoms/' and its adoption gave a tremendous 
impetus to the development of Organic Chemistry. 

These links or bonds joining the atoms together to form 
molecules are a pictorial representation of the affinities, in which 
it is assumed that a link stands for mutual saturation of the 
valency of one atom by the valency of another; and as the 
number of valencies for carbon is 4, whilst those for H, 0, N, 
etc., are respectively one, two, three, etc., it will be readily 
understood that all manner of formulae could be constructed on 
paper, which might or might not represent the actual molecular 
structure of real compounds. It is, therefore, perhaps not 
surprising that this wonderful theoretical instrument was 
destined to stimulate experimental investigation, which ulti- 
mately enabled chemists to build up synthetically a vast array 
of new carbon compounds and elucidate their molecular 
structure, as well as fix the constitution of hosts of naturally 
occurring substances. 

During all this great extension of Organic Chemistry it 
was found neither necessary nor profitable to enquire into the 
nature of the links, binding the atoms together in all these com- 
pounds, for the reason that until the twentieth century such 
enquiry was merely speculative, without experimental basis, 
and because chemists could get on perfectly well without such 
knowledge. And even now when the nature of the binding has 
been more or less " explained" by invoking electronic forces, 
chemists for the most part are content to preserve their simple 
pictorial, non-committal, structural formulae for ordinary use. 
At the same time it is desirable to state briefly what the new 
views are, as indicated by the discoveries in Atomic Physics 
and by X-ray analysis (p. 229). 

Bragg has shown how carbon atoms are disposed in the 
crystal of diamond (pure carbon), and this gives the key. The 
atoms are so arranged in the lattice or framework, that every 
carbon atom is surrounded symmetrically by four other carbon 
atoms. This is not in the least surprising, for it had long 
ago (1874) been deduced by le Bel and van't Hoff from 
purely chemical considerations. Indeed, a whole new branch 


of knowledge (stereochemistry p. 446) dealing with the 
arrangement of atoms in space had been built up on the 
simple assumption that the four valencies of carbon were 
directed symmetrically from the centre of a sphere, to the four 
corners of an imaginary tetrahedron (p. 27) inscribed within 
the sphere. 

Whilst, therefore, modern observations entirely confirm 
earlier theoretical conclusions, this does not help us in under- 
standing what the valency bonds really are, or how they bind 
the atoms so securely together in the molecules of compounds. 
In this respect we are still to some extent on speculative ground, 
but the modern theory is, that when atoms are singly linked a 
valency-electron (p. 251) of one atom somehow joins forces 
with a valency-electron of another atom, and the two electrons 
(moving, no doubt) form a pair between the atoms. It is this 
pair of electrons which constitutes a single link. Thus, to 
return to methane, CBLt, each of the four valency-electrons of 
the carbon atom joins up with the single valency-electron of 
four hydrogen atoms, forming four pairs of electrons, lying 
symmetrically about the carbon atom at the four corners of 
an imaginary tetrahedron. It still remains unexplained why 
this should be, or how these pairs preserve their mean positions 
while in motion; and, as will be imagined, the case becomes 
complex when less simple compounds are considered or when 
the so-called " double link " (p. 392) is present. We must 
leave this intricate problem and pass on to consider how, on the 
simple conception of linking of atoms, molecular structural 
formulae can be built up. 

Now if, say, six carbon atoms be linked together in a row 
or "chain," one valency (or link) each of contiguous atoms 
mutually " saturate " each other, giving the skeleton 


The terminal C-atoms will have three links left free to unite 
with hydrogen atoms, while the remaining four will have two 
only. When these fourteen valencies are saturated by 
fourteen hydrogen atoms, we obtain a structural formula 


showing how the six carbon and fourteen hydrogen atoms are 
linked together, in a molecule of the hydrocarbon, C 6 Hi 4 
(hexane) : 

V ; 

H H H H H H 
H C (LcU-cL-0~C H 

Such a type of formula clearly expresses the constitution of 
the compound, and therefore is called a constitutional or 
structural formula; whilst formula like C 6 Hi 4 , which merely 
express the numbers of atoms in the molecule, are called 
empiric formula. Hexane has a type of structure similar to 
that of a large number of carbon compounds, called the aliphatic 
series, including paraffins, fats, oils, etc. But many organic 
compounds are much more complex in structure, than this 
simple type (hexane) selected for illustration. Their synthesis 
has been effected step by step in such a way as to prove their 
constitution; and the elucidation of the actual mode of atom- 
linking in the molecular structure of hundreds of thousands of 
such compounds is indeed one of the noblest triumphs of 
modern science. There still remain however large numbers of 
complex bio-chemical compounds, whose synthesis has not yet 
been accomplished. 

Returning to methane and similar hydrocarbons, we can 
replace H by a group consisting of one carbon atom, two 
oxygens and one hydrogen, a combination group or " radical " 
known as " carboxyl," and get the so-called series of the fatty 
acids, each member of the series depending on the number and 
1 linkage-mode of carbon atoms present in it. Thus : 

H c c'' 

A X 

X H 

is acetic acid. If there are four carbon atoms in the chain we 
get butyric acid, the substance present in rancid butter; if 
there are sixteen carbon atoms, palmitic acid, the acid obtained 
from palm oil, and so on 

Or again, if we replace a hydrogen atom by a combina- 


tion group of one oxygen and one hydrogen (known as the 
radical hydroxyl, OH), we enter on the series of the alcohols. 


H H 

i JJC 

is ordinary alcohol, or ethyl alcohol, while the formulae, 

H H H H H H 

H C H and H C C C C C O H 

I I i ! I I 

H H H H H H 

give the structure of methyl alcohol and amyl alcohol respec- 
tively. The latter, however, is only one of several possible 
amyl alcohols, all C 5 H n .OH. For it is possible to link these 
atoms together in several different ways. This phenomenon, 
of different structure (and properties) for the same numbers of 
atoms in the molecule, is very frequent among organic com- 
pounds, and is known as isomerism. Isomeric compounds 
possess the same empiric formula, but a different constitution 
and different properties. 

The possibility of isomerism among organic compounds 
will be appreciated best by considering a simple case, such as 
the different modes by which four carbon atoms can be linked 
together, so as to satisfy the tetravalency of each atom. It 
will be readily seen that only two skeletal frameworks are 
possible viz. : 

(i) C C~C-~-C- and (2) 

I I I I -<p 

So that we can predict that there can be two hydrocarbons 
QHio, and only two, derived by the saturation of the ten 
available links in each case with hydrogen. Two such hydro- 
carbons are known, and only two : one whose structure is built 
up of a simple chain (i), and called normal butane; the 
other, called iso-butane, having the branched or side-chain 
structure (2). 

But a moment's reflection will show that the number of 
possible modes of atom-linking will increase rapidly as 


number of carbon atoms in the skeleton increases, and so, of 
course, the number of isomeric hydrocarbons possible. Indeed, 
the possible number becomes vast when there are say fifteen 
carbon atoms; and when it is considered that these hydro- 
carbons are the parents of numberless derivatives, obtained 
by replacing one or more hydrogen atoms by other atoms or 
groups, and that there are many alternative positions for 
these " substituents " to take up, it will be realised that the 
number of possible isomers among organic compounds ap- 
proaches the infinite. 

It is not necessary that the available free linkages, left 
over after joining the carbon skeleton together, should be 
satisfied by hydrogen. Pairs of unsatisfied valencies may 
appear as in the case of ethylene (C 2 H 4 ) or acetylene (C 2 H 2 ), 
which we can contrast with the saturated hydrocarbon, ethane 

(C 2 H 6 ) thus: 

v ' H H H H 

H-C-k-H H^cLcL-H H C~C~H 

i i ' ' '< ' 

Ethane Ethylene Acetylene 

Such types of compound and there are many of them are 
called unsaturated compounds, and it is usual to express the 
formula by a double link or triple link respectively, as if the 
free valencies saturated themselves mutually, thus : 
H H 

C=C and H OsC H 

This is a convenient device to signify unsaturation and the 
particular pair of contiguous carbon atoms (say in a long chain) 
which are unsaturated. Saturated hydrocarbons are called 
paraffins (Lat parum affinis) because they have all their 
affinities equalised, and so are very inert chemical substances 
compared to unsaturated compounds which, by tending to 
become saturated, are chemically reactive. 

In 1825 Faraday discovered a substance which he called 
" bicarbonate of hydrogen/' and which he obtained from a 
deposit in vessels used in the manufacture of gas from oil, 


with which the streets of London were, at that time, illuminated. 
This parent hydrocarbon, which we now call benzene, has six 
carbon and six hydrogen atoms in its composition, that is to say 
the empiric formula C 6 H 6 . Benzene in properties appears to 
be saturated, but it can be induced to accept six more 
hydrogen atoms, twelve in all, not fourteen as in hexane. 
This peculiarity remained a puzzle until 1865, when the German 
chemist, Kekul6, at that time working in the chemical labora- 
tory of St. Bartholomew's Hospital, London, hit on the idea of 
closed chain or ring structure. Thus by linking the ends of 
the skeleton formula of hexane, shown on p. 389 i.e., uniting 
the loose ends into a ring and by doubling the links of 
alternate carbon atoms he obtained the now well-known 
" benzene ring/' thus: 

This forms the basis of the molecular constitution of a host 
of carbon compounds known as the Aromatic Series which 
include aniline dyes, certain explosives, synthetic drags, etc. 
There is no difficulty in seeing how such a ring containing six 
carbon atoms may fix a total of twelve hydrogen atoms and no 
more, for all that is necessary is to add one more hydrogen atom 
to each carbon, by breaking the three double links and making 
them single. We then have two hydrogen atoms attached to 
each carbon giving the substance hexa-hydro-benzene (cyclo- 
hexane), when each carbon atom will be saturated thus: 

H H 




Ring structure or cyclic grouping leads to many other types of 
series; thus when a trivalent nitrogen atom is substituted for 
one of the carbon atoms in the benzene ring, we get pyridine 
as is shown in the formula: 

With these three ideas of skeleton structure in his hands (i) the 
carbon chain (Aliphatic Series), (2) the carbon ring (Homocyclic, 
including Aromatic Series) and (3) the ring containing carbon as 
well as one or more foreign atoms, like nitrogen (Heterocyclic 
series) it is possible for the chemist to build up, theoretically, 
all sorts of compounds, and also actually to synthesise them 
with wonderful diversity in the laboratory. 

General Reactions and Synthesis. It is necessary now 
to refer briefly to a few types of universal chemical change, 
because these general reactions are fundamental to chemistry, 
either for purposes of synthesis or in relation to life. 

In 1836 Dumas showed that a hydrogen atom of a paraffin 
could be replaced by a chlorine atom with production of hydro- 
gen chloride as a by-product. This type of reaction, known as 
substitution, the laws of which, as formulated by Dumas, met 
with a good deal of opposition at the time, applies also to 
chlorine or iodine. The simplest example of substitution 
can be written as an equation thus: 

CH 4 + C1 3 = CH 3 .C1 + HC1 

Methane Methyl chloride 

and by further substitution it is possible to replace the remain- 
ing hydrogen atoms, giving successively the compounds CH 2 C1 ? , 
CHC1 3 (chloroform), and CC1 4 . 

To illustrate the utility of substitution, let us take another 
simple case. Just as methane gives methyl chloride, so the 
paraffin ethane, C 2 H 6 , gives ethyl chloride, C 2 H 5 .CL Now as 
are get to more complex cases it becomes rather inconvenient 


to use constitutional formulae in equations, so rational formulae 
are generally used, these being, as it were, shorthand for the 
full structural formulae (generally using dots in the place of 
links, and omitting them when obvious). We can therefore 
write ethane CH 3 "CH 3 , and ethyl chloride CH 3 'CH 2 'C1, both of 
which rational formulae imply clearly enough how the atoms 
are linked. 

When ethyl chloride is treated with water (H'OH) in a 
suitable way, HC1 is formed, and the chlorine atom substituted 
by the hydroxyl group (OH), giving CH 3 *CH 2 *OH, which, as 
we have seen (p. 391), is common alcohol. Now this is a very 
simple but interesting thing, for we have thus in two steps 
synthesised an important organic compound; and since the 
starting-point (ethane) can be obtained from acetylene 
(CH CH) by suitable treatment with hydrogen, and since 
acetylene is formed from carbon and hydrogen at the tem- 
perature of the electric arc, as was first shown by Berthelot^ 
it follows that ethyl alcohol can be indirectly built up or 
synthesised from its elements. And what is true of alcohol is 
true of all organic compounds (several hundred thousand) 
which have been synthesised ultimately they can be built 
up from the elements composing them, and the steps by which 
this is done indicate their structure. 

A very notable difference distinguishes unsaturated from 
saturated compounds in their behaviour with chlorine, bromine, 
and iodine (the halogen elements) ; for instead of " substituting/' 
unsaturated compounds add a molecule of the halogen. For 
example, ethylene (p. 392) rapidly takes up chlorine, the 
double link becomes single, and ethylene dichloride CH 2 C1'CH 2 C1 
results. Mo|j5over, unsaturated compounds add many other 
substances as well as halogens, and this general reaction 
(addition) has proved of great service in synthesis, 

~&xi'dation is a general type of change implying either the 
taking up of oxygen (as, for example, when iron rusts and forms 
a hydrate of ferric oxide, Fe 2 3 ), or the removal of hydrogen 
(generally only partially). In organic chemistry oxygen gas 
(0 2 ) is not generally used, because it is not sufficiently active, 
but as oxidising agents such substances are used as readily 


part with some of their oxygen (like nitric acid, HN0 3 , potas- 
sium permanganate, KMn0 4 , etc.) . Thus by suitable oxidation 
ethyl alcohol gives first acetaldehyde (CH 3 *CH : 0) and then 
acetic acid (CH 3 *CO'OH), the first stage involving removal 
of hydrogen and the second the taking up of oxygen. In- 
cidentally, we have synthesised a fatty acid (acetic) which, 
like all organic acids, contains the typical group CO'OH. 
Oxidation plays a very important part in the chemistry of life, 
as we shall see later. 

Reduction, which is the opposite of oxidation, involves the 
taking up of hydrogen or elimination of oxygen. For example, 
when acetylene (p. 392) takes up four hydrogen atoms, it is 
said to be reduced to ethane, and all unsaturated compounds 
behave similarly; those with a double link take up two, and 
those with a triple link four hydrogen atoms. But this must 
not be taken to imply that ordinary treatment with hydrogen 
gas will cause such reduction; as a rule special so-called " re- 
ducing agents'* are employed, bodies which are themselves 
easily oxidisable in the process. Nevertheless, hydrogen gas 
can be used, as was indicated by Sabatier and Senderens, if 
suitable metals are present, like finely divided nickel, to 
activate the hydrogen and function as so-called catalysts. 
We will refer to catalysts again (p. 447), and for the present 
it is sufficient to say that they are agents which somehow 
hasten an otherwise sluggish chemical reaction. This kind of 
reduction with hydrogen, using metallic catalysts, is of great 
importance in the manufacture of margarine from oils. The 
latter contain derivatives (glycerides or fats) of fatty acids, 
some of which, like oleic acid, are unsaturated, and when they 
take up hydrogen by reduction, they give saturated fats 
which have a higher melting-point. By this process (" harden- 
ing of fats "), regulated carefully, oils are converted into fats 
of the consistency of butter. 

Condensation is a term implying the linking together of 
two molecules by eliminating something between them, gen- 
erally a molecule of water. We wiU illustrate this impor- 
tant type of synthetic reaction by considering the case of an 
acid and an alcohol, say acetic acid and ethyl alcohol. When 


these two are mixed together, nothing apparently happens 
at first, but after a while it is found that a new substance is 
slowly appearing, called ethyl acetate, which has a sweet, fruity 
odour. This substance is one of a similar class, called esters, 
derived by the elimination of H 2 between the OH group of the 
alcohol and the CO" OH group of the acid, and the formula of 
ethyl acetate is CH 3 *CO'OC 2 H 5 . The reaction leading to 
esters is greatly accelerated if there is present a little " mineral " 
acid, like HC1, which functions as a catalyst, but the change 
is never complete. Here we have an example of a reversible 
reaction, the nature of which is explained on p. 399. It will be 
noted that esters are derived from acids by the replacement 
of the carboxylic hydrogen atom by some group like C 2 H 5 . 
In this sense they resemble salts, like sodium acetate 
CH 3 "CO*ONa, and indeed, esters were once called ethereal 
salts. But they differ from salts by the fact that they are not 
ionised, and they are quite different in properties. The simple 
esters are found naturally in fruits and some perfumes, whilst 
those esters which are derived by condensation of glycerol 
(p, 436) and complex fatty acids are the natural fats and oils 
(glycerides), as we shall see later. 

Hydrolysis is a type of chemical reaction exactly the 
opposite of any condensation process involving elimination of 
water. That is to say, as the name indicates, hydrolysis means 
the splitting of any compound into simpler molecules by the 
assimilation of the elements of water, Suppose, for example, 
we take some pure ethyl acetate and shake this sweet-smelling, 
colourless, mobile liquid with pure water. A portion of it will 
dissolve, giving a neutral solution, and apparently nothing else 
happens; but, on standing a long time, it will be found that the 
watery solution is no longer neutral; it is acid, so turns litmus 
red, and this acidity, due to production of acetic acid (along 
with alcohol) increases with time. The hydrolytic fission can 
be written: 

: 1 

H'OH = CH 3 COOH + CH 3 'CH a -OH. 

And again here, as before with condensation, the velocity 
of the change is greatly increased by having present a little 


strong inorganic acid, which (mainly by its hydrogen ions) 
functions as a catalyst and remains unaltered at the end. 

Now here it may be remarked that there is something 
contradictory. How can, say, hydrogen chloride favour two 
opposite reactions (condensation and hydrolysis) at the same 
time ? To clear up this apparent difficulty, let us dispense 
with the hydrogen chloride, which, after all, is not necessary 
to either change, but only hastens both, and consider the slow 
opposite reactions themselves. The key to the difficulty is 
found in the recognition of mass influence. This was first 
clearly demonstrated by the researches of Guldberg and Waage 
in 1867, when they showed that many chemical reactions, such 
as "double decomposition " with salts, are reversible, and that 
the extent of change in either direction is dependent on the 
mass f f6t> number of molecules. Consider, for example, 
the case of two salts mixed in strong aqueous solution, say 
sodium nitrate and potassium, chloride, where double decom- 
position gives rise to potassium nitrate and sodium chloride in 
the reversible change : 

NaN0 3 + KCl=>NaCl + KN0 3 . 

In the solution we have, after mixing, all four salts present 
(apart from the four ions Na+, K+, Cl~, and N0 3 "~), and the 
extent to which the reaction proceeds in either direction 
(indicated by arrows) is determined by the number of the 
reactant molecules in unit volume i.e., upon their concentra- 
tion. This is the meaning of Guldberg and Waage's mass law, 
which amounts to this, that the velocity of any reaction is 
partly governed by the concentration of the reacting substances. 
It must always be remembered that we are not dealing with 
single isolated molecules, such as appear in an equation on 
paper, but with myriads of them ; and large battalions conquer. 

For example, let us suppose in the above case that the 
solution is so strong (or, say, is cooled from a hot, strong 
condition) that the least soluble salt, which is potassium 
nitrate (KN0 3 ), separates out as crystals, because there is not 
sufficient water to hold it all in solution. This would cause a 
decrease in the concentration of one of the components on the 


right-hand side of the equation, and so lower the velocity of 

the change in the direction shown ; therefore more NaNO 3 

and KC1 would go over in the direction shown -> as KN0 3 
separated out. 

We can thus manufacture potassium nitrate (saltpetre) 
from sodium nitrate (Chili saltpetre) and potassium chloride; 
and there are many similar applications of this principle, 
such, for instance, as making all manner of insoluble salts by 

Such reversible reactions of double decomposition are 
practically instantaneous, because they are between ions, 
which are formed with extreme rapidity from molecules of 
salts. This is not the case with acetic acid and alcohol, but 
the reaction, which is slow, can be written as a reversible 

equation of equilibrium : 


CH/CO-OH + C 2 H,;OH;~I^CH 3 -CO-OC 2 H5 + H 2 O. 

Acid and alcohol hydrolysis Ester and water 

Both direct and reverse reaction are very slow. Even under 
the catalytic influence of a strong acid they are slow, compared 
to the ionic velocities of double decomposition. Nevertheless, 
the mass law holds good; the velocity in either direction is 
determined by (i) an affinity factor which is constant, and (2) 
the reacting masses, which change with time as the concen- 
tration (i.e., number of reacting molecules in a given volume) 
changes. At first, starting from the left-hand side of the 
equation, the concentration of acetic acid and alcohol will be at 
a maximum, and so the reaction in the direction > will be 
relatively rapid ; but as their concentration inevitably diminishes 
whilst that of ester and water increases, this velocity diminishes 
whilst that in the direction < increases. After a long time 
it must come about that the velocities of the opposing reactions 
are just equal; and when this is the case, equilibrium is es- 
tablished, in which there are as many molecules of acid and 
alcohol being condensed in unit time as there are of ester being 
hydrolysed by water. On the other hand, if an ester is hydro- 
lysed in presence of an alkali, the process is complete, because 
the alkali neutralises the acid as fast as it is formed. This is 


the well-known process of " saponification " by boiling with 
caustic soda, so called because it has long been in use (since 
ancient times) in the splitting of fats into glycerol and fatty 
acids as their sodium salts (soap see p. 437). 

Hydrolysis or saponification (the terms nowadays usually 
connote the same thing) is also of great importance in bio- 
chemistry, since, as we shall see later, it underlies the complex 
changes involved in the digestion of food. 

Diffusion, About the beginning of last century John 
Dalton made the interesting discovery that if he filled two 
bottles, one with hydrogen and the other with carbon dioxide, 
and united them by means of a long glass tube, the hydrogen 
bottle being uppermost, the lighter gas, hydrogen, passed 
gradually into the lower bottle and the much heavier carbon 
dioxide ascended into the hydrogen one, and that this diffusion 
continued until the two gases were equally mixed in both vessels. 
Dalton concluded that the gaseous particles were in constant 
motion, striving to get as far away from each other as possible, 
each quite indifferent to the presence of the other. The 
" atoms " of the gas were conceived as bombarding the walls 
of the containers in their efforts to escape, thus creating a 
pressure on the walls. On this conception rests the " kinetic 
theory of gases'' (p. 224). In 1859 Clerk Maxwell calculated 
the speed at which the gaseous molecules were moving, and 
deduced the pressure of a gas on the wall of the container 
under varying conditions, thereby enabling us to understand 
the mechanism of diffusion. 

As by the kinetic theory the average speed of the molecules, 
for a given temperature, is inversely proportional to the square 
root of the molecular weight, it follows that the speed of diffusion, 
say through some porous membrane through which the mole- 
cules can pass, will thus depend on the weight of the molecules, 
since diffusion depends on the rapidity of their movements. 
And since, as we have seen (p. 242), the density of a gas (at any 
definite temperature and pressure compared to hydrogen) is 
equal to half its molecular weight, it follows that the rate of 
diffusion of a gas, for any definite temperature and pressure, 
is inversely proportional to the square root of its density. This 



is Graham's law of diffusion, from which, of course, it follows 
thai hydrogen has the highest diffusion rate of all gases; but at 
the same time it must be remembered that the 
rate of diffusion of any gas increases with rise * 
of temperature, because, as we have seen, the 
molecular speed is increased. 

Diffusion also takes place between liquids,but, 
as might be expected from the kinetic theory, far 
more slowly than with gases, because the attrac- 
tive forces between the molecules of each liquid, 
and their smaller " free path," make it more diffi- 
cult for them to insinuate themselves between 
each other. If a jar is half filled with water 
(Fig. no, W), and a fine silk cloth, S, is pushed 
into the jar so as just to touch the water, and FIG. no. 
then a dilute solution of an aniline dye, E, be DlFFUSION OF 
very gently poured on the silk, the cloth being Q 

slowly withdrawn as the coloured solution is added, it will be 
found that the water below and the colour will remain apart for 
some considerable time. Gradually, however, the liquid in the 
jar will assume a uniform tint throughout, pointing to a slow 
mutual diffusion between the water and the dye solution. 
Care must be taken not to shake or otherwise disturb the 
jar, and to keep it at a uniform temperature, otherwise con- 
vection currents will accelerate the mixing. 

Strange as it may seem, diffusion may also take place be- 
tween solids. If, for example, a plate of copper and one of 
lead be firmly clamped together, so that the surfaces of the 
two metals are in the closest possible contact, it will be found 
that, after a year or more, the molecules of the copper and lead 
have intermingled at the plane of union. Diffusion in fact is 
very slow in the solid state, more rapid with liquids and most 
rapid with gases, as would, of course, be expected on the 
kinetic theory. 

Solution and Adsorption. But solid substances dissolved in 
liquids readily diffuse owing to the movement of their molecules. 
If, for example, strong brine (salt solution in water) is covered 
carefully with pure water, the latter being lighter floats on 



top, but in time tie salt from below will diffuse upwards into 
the water, and ' eventually the solution will have the same 
strength (concentration) all over. The process of diffusion 
does not stop here, the molecules still keep moving about 
though there is nothing now to indicate it, of course; it is 
important to recognise this incessant movement of dissolved 
molecules, as so-called " solute/' among moving molecules 
of solvent, a movement which prevents these molecules 
separating out and sinking to the bottom, if they are heavier 
than those of the solvent, or rising to the top if they are 

An important reservation, however, must be made here 
with regard to the movement of solute molecules, which in any 
solution (say in a closed bottle) keeps the concentration un- 
changed throughout its bulk, no matter how long it may stand. 
It is that, strictly speaking, this uniformity of concentration 
only applies to the bulk and not to the surface of contact with 
another solid (say the glass of the bottle) or with the air. 
Speaking generally, there is an increased concentration or local 
assembly of solute molecules at such surfaces, in layers which 
are only " skin deep " as it were, generally a single layer of 
molecules. But this increased concentration at surfaces, which 
is so small a thing in a bottle of solution, becomes of enormous 
importance if the surf ace happens to be very great; as it would, 
for example, if, instead of glass walls merely, we had suspended 
in the solution some solid powdered so finely that the particles 
had a diameter of only say *oi //, (see p, 259). 

Or again, to consider another aspect of the same problem, 
if instead of having a flat surface at the top exposed to air, we 
churn it up with air so as to make a real froth like soap lather, we 
greatly increase the surface between air and solution. This 
increased surface due to subdivision will be better appreciated 
by the reader if he imagines a cube, each of whose twelve 
edges is 2 inches long, divided by two transverse cuts into 
smaller cubes each of i inch edge. There will be eight of such 
smaller cubes, each having a face of I square inch. Now there 
are six square faces to every cube, and each face of the original 
large cube has, of course, an area of 4 square inches (2x2), 


so that our large cube has a total area of 24 square inches (6x4). 
Similarly each of the eight smaller cubes has an area of 6 
square inches, so that the total for all of them is 48 square 
inches (8 x6). So that on dividing our larger cube by two 
transverse cuts we have doubled the total original area. The 
same kind of thing applies to all subdivision, as, for example, 
when we powder a solid substance by grinding it in a mortar 
or mill; and when the grinding is very fine there is an enormous 
increase effected in the total surface. When the state of sub- 
division is such as to give small particles beyond the power of 
ordinary grinding, such as have a diameter of say "02 ^, the 
increase in surface is really prodigious, so great in fact that 
the phenomena we are now describing predominate. In 
such cases the local concentration at these extended surfaces, 
called " adsorption/' may be so great that very little of the 
solute is left in true solution at all, most of it being con- 
centrated on the scattered particles, whether solid or gas. 

The whole subject of adsorption has been very thoroughly 
worked out and formulated into mathematical expressions 
which we cannot consider here. It is sufficient to say that the 
phenomenon is intimately connected with that of surface tension, 
a condition of skin-like stretch of the molecules of liquids at 
their free boundaries. A]! liquids show such a skin of stretched 
molecules at the boundary surface in contact with air, and this 
skin enables certain insects to walk freely on the surface of 
sheets of water, or may prevent a needle lightly laid on the 
surface from passing through and sinking. Different liquids 
have different values for surface tension, which, moreover, 
may be altered by temperature changes or by dissolving certain 
substances in them. Most salts increase the surface tension 
of water, while soap reduces it ; and, in fact, it is this reduction 
which gives soap its cleansing (detergent) qualities, by en- 
abling oily films to be dispersed into minute emulsified drops, 
owing to the reduction of the tension between aqueous and oil 
layers. In the living cell, too, there are extended surfaces pre- 
sented by the colloids present (p. 405), and the varying surface 
tension here plays a great part in biochemical processes. These, 
however, are highly complex and beyond the scope of this book. 


Crystalloids and Colloids. If a soluble substance like potas- 
sium nitrate is added to water it dissolves, and this means that 
free-moving molecules of the nitrate, as " solute/' are distributed 
among the water molecules, as " solvent/' If more potassium 
nitrate is added the dissolution proceeds, until a time arrives 
when it takes place more and more slowly and finally ceases. 
The solution is then said to be " saturated/' If heat is applied 
the dissolution begins again, until, when the liquid is boiling, it 
again ceases, and another saturation point is reached. As the 
liquid cools the reverse process occurs, and crystals of potassium 
nitrate separate out; and if the solution is left to evaporate, 
all the salt (potassium nitrate) that was originally added to the 
water will crystallise out unchanged. 

It should be recalled here that a salt (p. 378) in the inorganic 
chemical meaning of the word, is a substance derived from an acid 
by replacing the ionisable hydrogen of the acid by some metal. 
Thus nitric acid, HN0 3 , yields nitrates as salts, like potassium 
nitrate, KN0 3 ; sulphuric acid, H 2 SOd two classes of salts, one, 
acid sulphates like NaHS0 4 and the other neutral or normal 
sulphates, like Na 2 S04, where Na stands for an atom of sodium. 
The most characteristic generic feature about salts is their 
ionisability when dissolved in water, a property that enables 
their solutions to conduct electricity and function as " electro- 
lytes." Just as an acid in solution is partly ionised i.e., split 
up into positive H-ions (hydrions) and negative ions (anions), 
so salts are (more completely) ionised in solution into metal ions 
(cations) and the same anions as the corresponding acid. A 
solution of potassium nitrate therefore contains not only 
molecules of KNO 3 , but also ions of K (each an atom which 
has lost an electron) and ions of N0 3 (groups of atoms each of 
which groups has gained an electron). This process of ionisa- 
tion increases as the dilution increases, and is regarded as com- 
plete at infinite dilution. The ionic theory of solution founded 
by van't Hoff and Arrhenius in 1886 was the outcome of experi- 
ments on osmosis described on p. 410, but it came as a great 
shock to chemists at the time, all of whom even at the present 
day do not accept the theory in its entirety. 

Early in the nineteenth century it was recognised that 

FIG. in. 



there was a very marked difference between the diffusion-rate of 
substances, which could assume a crystalline form in the solid 
state, and those which did not crystallise. The first to study 
the diffusion of soluble substances was Thomas Graham, who 
was professor of chemistry in University College, London, from 
1837 to *&55> w ^ en he took over the duties of 
Master of the Mint. In 1849 ^ e ave an account 
of his discoveries, in which he showed that certain 
substances in solution passed readily through an 
animal or vegetable membrane, such as bladder 
(by dialysis), while others did not do so, or did 
so extremely slowly. The first of these two kinds 
of substances Graham termed " crystalloids " and 
the second ' ' colloids. ' ' The crystalloids included 
all bodies capable of taking on a crystalline form 
in the solid condition, while colloids so named 
from the Greek word kolla, or glue were sub- 
stances like gelatine, albumin, etc., which swelled 
up in water, but never actually formed a solution. Graham's 
apparatus for demonstrating the characters of these two types 
of bodies is known as a dialyser (Fig. in), and consists of a 
jar, B, containing pure water, D, in which is suspended a bell- 
jar, A, whose bottom is composed of bladder or parchment, 
containing the colloid or crystalloid, E. 

Sols and Gels. Since 1900 immense progress has been 
made in elucidating the nature of colloids in solution, and this 
has been mainly due to the researches of Wolfgang Ostwald 
since 1908, following the invention of the ultra-microscope by 
Zsigmondy about 1903. It had long been known that while 
solutions of crystalloids were optically clear, those of colloids 
were either opalescent or turbid, and as such they were not 
regarded as very interesting by the chemists of the nineteenth 
century, who mostly avoided them in their zeal for dealing 
with pure substances. It was also known that such colloidal 
solutions did not clear by settling when allowed to stand, 
like ordinary suspensions, that some of them were slightly 
viscous, and that many such turbid solutions could be cleared 
when coagulated by addition of certain electrolytes. 


It was not until the twentieth century when study was 
directed to small magnitudes that their nature was elucidated 
by the extension of the kinetic theory, and an entire new field 
of physical chemistry opened up. The limit of resolving 
power of the best microscope is about '2 ^ ('0002 mm.). The 
ultra-microscope depends on transverse illumination and renders 
visible, as points of light scattered by particles optically differ- 
ent from the liquid in which they are suspended, particles as 
small as '005 /*, which is only about ten times the diameter of 
average molecules. In fact, turbidity, which is the result of 
optical scattering by such small particles, and regarded once 
as rather an unpleasant thing to be avoided, became the very 
means by which the nature of colloidal solutions could be 

And Brownian motion (p. 203), once so puzzling, was at 
last recognised to be the erratic zig-zag movement of very small, 
but relatively large, suspended particles incessantly bombarded 
by the still smaller molecules of the solvent, endowed with the 
kinetic energy which, as we have seen (p. 229), all liquids 

And so it came to be recognised that there was really no 
sharp dividing line between suspensions of large particles, on 
the one hand, in a liquid, and small ones, on the other it was 
all a matter of particle size. There is an important law, first 
formulated by Stokes in 1850, governing the rate of fall of any 
spherical particle through a gas or liquid medium, and the 
important point about this law is that, other things being 
equal, the rate is proportional to the square of the radius. So 
that, for example, if a particle of small radius fall i millimetre 
per second in a given liquid another particle of the same sub- 
stance y^-of its diameter would fall at the rate of -nj-Jmr milli- 
metre per second. This is negligibly slow, but nevertheless 
not infinitely slow, as it is with colloidal solutions i.e., suspen- 
sions of exceedingly small particles. There are, in fact, other 
forces at work besides Stokes* law in such cases viz., (i) con- 
tinual bombardment of the particles by the liquid molecules, as 
we have seen, and (2) electrical charges (either all + or all -) 
on the particles; these by repulsion tend to keep them apart. 


This last factor, which was unsuspected at first, is indeed 
the principal one which prevents the particles coming together, 
by those attractive affinities which were discussed (pp. 227, 228) 
in considering kinetic energy. The moment the electrical charge 
is destroyed or neutralised in anyway, say by adding the right 
kind of electrolyte (charged ion), the particles tend to agglomer- 
ate together into larger particles (coagulation or clotting) , which 
then settle more or less rapidly by sedimentation. 

Further research has made it clear that there are two very 
distinct kinds of colloid solutions or " sols," as they are now 
called viz: 

(1) Suspensoids. 

(2) Emulsoids. 

(1) Suspensoids are, as their name indicates, suspensions 
of excessively fine solid (and therefore crystalline) particles 
such as metals like gold and platinum, which can be got into 
" solution " in water, as permanent sols, by means of a powerful 
electric discharge under the water. 

(2) Emulsoids, as their name indicates, are suspensions of 
excessively fine oil drops in water or aqueous solutions, or 
conversely of fine water drops in oil solutions. 

In each case the suspended matter is said to be in the 
disperse phase, while the solvent is the continuous phase; but 
it is to be noted that it is among group (2) that high viscosities 
are usually observed, owing to mutual friction between the 
two liquid phases, exactly like what is observed in ordinary 
emulsions. Indeed, there is no sharp line of demarcation 
between emulsoid sols and ordinary emulsions, such as are 
got by shaking an oil with water, containing a little soap or 
something else which prevents the oil drops from coalescing 
together. Milk is a natural emulsion in which the particles 
of fat (butter) are of moderate size, greater than in emulsoid 
sols. So the cream slowly rises to the surface, being lighter 
than the solution of calcium caseinogenate, milk sugar (skim 
milk) etc., in which it is suspended; and cream itself is a 
fairly stable emulsion which is only with difficulty made to 
coagulate or clot into butter by churning (facilitated by 


hydrogen ions due to lactic acids, produced by the fermenta- 
tion of milk sugar see p, 445)- 

In addition to sols, where usually the disperse phase is 
relatively small in amount, there are gels, where it is relatively 
great and the continuous phase small. This gel (j elly) condition 
appears in the coagulated particles precipitated from sols; but 
it is more important in colloids like gelatine and biological 
cellular membranes, including food, leather, wood, etc. Here 
the discrete particles are not free, as in sols, but clotted together 
in the form of fine threads, network, or in some other way, in a 
disperse phase (water), whose amount may be relatively large 
or small. 

It is worthy of note that the disperse and continuous phases 
responsible for colloidal phenomena are not necessarily confined 
to solids and liquids. A logical extension of the conception 
must naturally include froths and smokes. Froths and foams 
consist of excessively fine air particles, as a gaseous disperse 
phase, suspended in a liquid continuous phase; while smokes 
consist of fine solid or (more frequently) liquid particles 
suspended in air. In common smoke from coal fires the dis- 
perse phase is a complex liquid mixture (tar) in an exceedingly 
fine state, and accordingly kept in suspension by the kinetic 
energy of the air molecules, as well as by electrical charges; so 
that smoke-polluted air doesnot reallyclarify itself, and we must 
depend on rain for purification in the long run. Even pure air 
has been found to have an enormous number of unsuspected 
small particles suspended in it (fine solid debris, or dust, and 
evaporated salt particles from sea spray), as well as bacteria 
spores, etc. ; and the optical qualities of the atmosphere (haze 
and visibility) partly depend on the number and nature of such 
particles, which serve as centres of adsorption (see p. 403) of 
water molecules contained in moist air. 

An immense amount of knowledge has now been accumu- 
lated concerning colloids and surface tension phenomena, 
which we cannot even summarise here. It will be sufficient 
to say that this wealth of knowledge permeates into every 
aspect of biology, and the industries founded on it, because 
the three prime classes of chemical compounds associated with 



the living cell viz., the carbohydrates, the fats, and the 
proteins function in the juices of the cell (whether vegetable 
or animal) as colloids and emulsions. And as our knowledge 
increases it is becoming more and more evident that chemical 
changes can occur at the greatly extended surfaces of colloid 
particles, such as would scarcely be dreamt of as possible in 
ordinary solutions. In these peculiar surface reactions, of 
which as yet relatively little is known, probably lies the key 
to many of the mysterious chemical manifestations of living 

Osmosis. Diffusion through a colloid membrane is some- 
thing like separating small ashes from larger cinders with a 
riddle. The membrane acts as a sieve, 
allowing only smaller molecules or ions to 
pass through the network of its disperse 
phase (embedded in a continuous water 
phase); and the phenomenon of such diffu- 
sion through a colloid membrane is known 
as osmosis. It is of profound significance 
in the physiology of the living organism. 
The names associated with researches on 
osmosis are those of Pfeffer, professor of 
botany in Leipzig, and van't Hoff, professor 
of physics, first in Amsterdam and then in 

Pfeffer's method, described in a paper 
published in 1877, consisted in using what is called a " semi- 
permeable membrane/' When a drop of a solution of copper 
sulphate is brought in contact with a solution of potassium 
f errocyanide, a reaction takes place between them which results 
in the formation of a colloidal film of copper f errocyanide on the 
surface of the drop, which has all the characters of a semi -perme- 
able membrane permitting the passage through it of some sub- 
stances but not of others. The pellicle, however, is too delicate 
for experimental purposes, so Pfeffer hit on the plan of using 
a porous earthenware pot, in which he placed a solution of 
copper sulphate. He then plunged the pot into a solution of 
potassium ferrocyanide, with the result that a precipitate of 

FIG. 112. OSMO- 



copper ferrocyanide was formed, in the pores of the wall of 
the pot (Fig. 112). After rinsing out the pot he placed in it a 
10 per cent, solution of cane-sugar, S, and sealed it securely 
with an air-tight rubber stopper through which was fitted a 
manometer, M. The pot was then immersed in water, W, 
which at once began to enter through the wall, and the sugar 
solution, thus diluted, ascended the inner leg of the mano- 
meter, causing the mercury to rise in the open leg, thus indicat- 
ing the pressure exerted by the sugar solution on the wall of the 
pot. This is called " osmotic pressure/* 

From the large number of measurements which Pfeffer 
made on the relative osmotic value of a variety of substances, 
van't Hoff, in 1887, demonstrated an exact correspondence 
between osmotic pressures in solutions and gas pressures. 
This osmotic pressure was found to be directly proportional to 
the strength of the solution, but it was observed, at the same 
time, that when the dissolved solute was an electrolyte (p. 255), 
especially if the solution was very dilute, and the molecules 
underwent ionisation (p, 378) , each ion had the same osmotic 
value as an entire molecule. Thus, if the substance were 
potassium nitrate, KN0 3 , the molecule splits into the ions 
K + and N0 3 ~, the first electro-positive and the second nega- 
tive, and each is equal in osmotic value to an entire undis- 
sociated molecule, KN0 3 . 

On the other hand, a substance, like cane sugar, which is not 
an electrolyte, behaves normally in solution, that is to say the 
osmotic pressure simply depends on the number of molecules; 
so they are not split up (ionised) and do not conduct electricity. 
It was on this foundation of the study of osmotic pressure and 
electrical conductivity that van't Hoff built up the modern 
ionic theory of solution and was able to calculate the degree of 
ionisation of electrolytes (i.e*> acids, bases and salts) at various 
concentrations; and the one salient fact emerged, that the dis- 
tinguishing feature of all acids is their production of hydrogen 
ions in solution the stronger the acid the greater the con- 
centration of H-ions. A hydrogen ion is simply a hydrogen 
atom which has lost its electron. It is therefore a proton 
(p. 245) but not a free proton, since in some way it is combined 


with the water molecules of the solution in which it is 

Osmotic pressure increases with temperature, exactly as 
volume or pressure does with gases (law of Gay-Lussac and 
Charles, 1802). So that here in osmotic pressure we have an 
exact analogue in solution of what is happening to gases, as 
postulated by the kinetic theory. The dissolved molecules 
(say of cane sugar) exercise the same kinetic energy as if they 
were existent as a gas having the same volume and temperature 
as that of the solution. Just as the pressure of a gas is a 
function of its temperature and concentration (i.e., number 
of molecules in unit volume), so the osmotic pressure of any 
un-ionised dissolved substance is the same function of the 
temperature and its molecular concentration. 

van't Kofi's discovery was of vast importance to chemistry, 
for it demonstrated clearly that Avogadro's law of gases, which, 
as we have seen (p. 242), is the most fundamental thing in 
chemistry, is equally applicable to solutions. Therefore, just 
as Avogadro's law was the only means by which the molecular 
weights of gaseous substances could be arrived at (or substances 
which could be vaporised so that their gas density could be 
determined), so this extension of the law to solutions enabled 
the molecular weight of such substances (like cane sugar) to 
be determined (by their osmotic pressure) as could not be 
vaporised without decomposition. As this applies to large 
numbers of organic compounds, this law of osmotic pressure, 
together with a law discovered by Raoult, based upon it and 
relating the depression of freezing-point of a solvent to molec- 
ular weight of solute, gave an enormous stimulus to Organic 
Chemistry. Moreover, these laws provided a similar stimulus 
to physical chemistry, by furnishing an exact means through 
which the degree of molecular dissociation (ionisation) in 
solution could be calculated, in the case of acids, salts, and 

Let us consider two simple examples of osmosis, one taken 
from biology and the other from an industrial process 

(p. 414). 

It is a well-known fact in vegetable physiology that every 


plant contains, on ultimate analysis, twelve chemical elements 
combined in various ways to form proteins, carbohydrates, 
fats and so on. These elements are, using their chemical 
symbols for the sake of brevity, C, H, O, N, S, P, K, Ca, Mg, 
Fe, Na, CL Many other elements certainly do occur in plants, 
but these, with relatively few exceptions, are accidental, and not 
essential to the plant's well-being. Of the twelve elements 
mentioned, C, H, and are by far the most abundant, as the 
following table shows (Eberaiayer, 1882) : 

Wheat Grain. C, 46-1; H, 5-8; O, 43-4; N, 2-3; ash, 2-4. 
Peas.C, 46-5; H, 6-2; O, 40-0; N, 4-2; ash, 3-1. 
Potatoes. C, 44-0; H, 5-8; O, 44-7; N, 1-5; ash, 4-0. 
Eye Straw. C, 49-9; H, 5-6; 0, 40-6; N, 0*3; ash, 3-6. 

(It should be noted that these figures are derived from an 
analysis of 100 parts of the plant substance dried at 100 C., 
so that all the uncombined water had been evaporated before 
the analysis was carried out.) 

Carbon and oxygen are elements obtained by the plant from 
the air, all the rest being derived from soil-water. The acquisi- 
tion of nitrogen from the air by some plants is a special case 
which is dealt with under biology (p. 418). The nitrogen in 
ordinary cases, and other elements (in the ash) are derived from 
nitrates, phosphates, etc,, dissolved in the water absorbed by the 
roots, and an analysis of soil-water shows us that the solution, 
presented to the root, is exceedingly dilute viz., about 10 to 
15 parts of solid to 10,000 of water, or 0*1 to 0*15 per cent. 
The conditions are thus favourable to ionisation. In order to 
obtain the necessary salt ions the plant must therefore absorb 
far more water than it really requires, and the excess must be 
got rid of by what is called in vegetable physiology " tran- 
spiration/ 1 which may be defined as evaporation under proto- 
plasmic control. 

The agents, for the absorption of the dilute solution from 
the soil, are the root hairs which cover the terminations of 
the ultimate rootlets. These may be easily seen if some mustard 
seed is sown on a piece of moistened cloth or blotting-paper 
and examined after two or three days. The microscope shows 




us that each root hair (Fig. 113) is a tube whose wall is a delicate 
membrane of cellulose, C, lined by a layer of protoplasm, P, 
and enclosing a cavity, V, more or less filled with a watery solu- 
tion of various organic and 
inorganic substances, called 
"cell-sap," and having a 
concentration approximately 
ten times as great as that 
of the soil solution. In the 
root hair we have a natural 
dialyser, for the cellulose wall 

and the protoplasmic lining are both semi-permeable membranes, 
permitting the entry of the soil solution, but resisting the exit of 
the organic compounds in the sap. The consequence is a continued 
inflow of the dilute soil solution into the sap cavity, and a steady 
increase in osmotic pressure within the root hair. Excessive 
distension, and rupture of the hair, are averted by the withdrawal 
of the solution by cells in continuity with it, thus facilitating the 
entry of more soil solution. Further, the osmotic pressure 
keeps all the living cells distended or in a state of " turgidity," 
which is essential to cell growth. If a young growing cell be 
placed under the microscope and surrounded by a 4 per cent, 
solution of potassium nitrate, 
the cell will be seen to shrink 
(Fig. 114, 2) . The explanation is 
that the natural conditions are 
now reversed; water is being 
withdrawn from the sap, and 
the elastically stretched cell 
wall contracts. If the external 
solution be raised to 6 per cent. (3) the protoplasm withdraws 
from the wall and the cell is then said to be in a state of " plas- 
molysis," and growth of the cell must cease since the con- 
structive agent, the protoplasm, is no longer in contact with 
the cellulose membrane. If the external solution be raised to 
10 per cent, (4), the protoplasm aggregates into a compact ball 
in the centre of the cell. The phenomena of plasmolysis were 
made great use of by Pfeffer and De Vries in the closing years 





of last century when estimating the osmotic equivalents of 
various substances. 

Bound up with osmotic phenomena in relation to protoplasm, 
is the no less important bearing of H-ion concentration on life- 
processes. Whether in the cell sap of plants, or the plasma 
of the blood of animals, or in water which sustains aquatic life, 
the hydrion concentration, or degree of acidity, is a controlling 
factor of supreme importance. Pure water, itself, dissociates 
(ionises) to a faint degree giving positive H-ions and negative 
ions of the group OH. Addition of alkali increases the concen- 
tration of the OH-ions, and reduces that of the H-ions to an 
infimtesimally small amount. Such liquids are alkaline, like a 
solution of caustic soda (NaOH). Addition of acids to water 
increases the H-ion concentration, but in all cases the product 
of the two concentrations (i.e., of H multiplied by OH) is 
constant, and the same as the product for pure water. The 
symbol pH is now used in all branches of science to indicate the 
H-ion concentration of any liquid, but as the symbol involves 
a mathematical conception it need not be further explained. 
It is sufficient to say that in most natural fluids the H-ion 
concentration is very low but variable, according to the part 
which the fluid plays in biological phenomena, such as 
respiration, digestion, muscular effort, plant assimilation, etc., 
and how this variation is effected is one of the wonders of 

Taking next an illustration from industry, it is well known 
that one of the chief sources of sugar is beetroot, and the 
problem before the manufacturer is to extract the sugar from 
among the host of albuminous and other colloidal substances 
present in the sap. Beetroot sap contains from 14 to 18 per 
cent, of sucrose i.e., cane-sugar but the presence of all these 
other bodies interferes with its purification and crystallisation. 
The method adopted is to cut the root into thin slices and steep 
these in hot water, when the sugar and other crystalloids dialyse 
out into the water, leaving the colloids behind in the cells. By 
passing the extract through a series of tanks the solution is 
gradually concentrated, after which it is treated with certain 
chemical reagents which cause precipitation of all the con- 


stituents save the sugar. The sugar solution, kept at the 
right pH, is then filtered and finally evaporated in vacuum 
pans, when the sugar itself finally crystallises out. 

Applications of Modern Chemistry. If we found it difficult 
to treat of the applications of modern discoveries in physics 
in the space at our disposal, it is simply impossible to perform 
the same service for chemistry. We must content ourselves 
with merely mentioning some of the chief lines in which recent 
discoveries have been applied to industrial processes. 

In metallurgy, although the extraction of metals from ores 
is still carried out on the principles followed by the ancients, 
many improvements have been made and new methods intro- 
duced, such as electrolysis and very high temperatures in the 
separation of the rarer metals. Then there is the Bessemer 
process, of converting iron into steel, and its modern develop- 
ments, and the utilisation of the slag as a source of phosphates for 
agricultural purposes; the employment of the rare earth oxides 
in the manufacture of gas mantles, and of potassium cyanide 
in the extraction of gold and silver from their ores. The old 
method of obtaining oxygen by the use of barium oxide has 
been supplanted by the fractional distillations of liquid air. 
Round the metal sodium cluster a whole series of industries, 
such as the manufacture of caustic soda and bleaching powder. 
The various alcohols, phenols, acetic acid, ethereal oils and 
essences, occur as groups of substances in the manufacture of 
which modern chemistry is deeply concerned. 

When we add soap manufacture, the preparation of petrol, 
the so-called ' ' cracking ' ' of oils and the transformation of heavy 
petroleum into petrol, the fixation of atmospheric nitrogen 
by the several processes, the problems connected with the 
making of synthetic rubber, the huge aniline dye industry, 
following on the discovery of mauveine, the first coal-tar dye 
product by Sir W. H. Perkin in 1856, the synthesis of indigo 
by Baeyer in 1878, and, lastly, the manufacture of explosives, 
beginning with nitro-glycerine by Nobel in 1869, and ending 
with "T.N.T." (trinitrotoluol), amatol and tetryl during the 
Great War when we think of all these and many other evolu- 
tions and applications of chemical research that have taken 


place during the past fifty years, we must realise that any 
adequate account of even one section of them would require 
a volume in itself. 


If we look back over the history of biology in the nineteenth 
century we cannot help being struck by the magnitude of the 
changes that took place in the science during the period 
1845-70. It was in the course of these years that protoplasm 
came to be recognised as " the physical basis of life/' both in 
the plant and in the animal; our knowledge of the lower forms 
of organic nature had been vastly extended as the microscope 
became an increasingly efficient instrument of research, and as 
microscopic technique steadily improved; vital phenomena 
received new interpretations as the chemistry of what came 
to be called " metabolism " received more and more attention; 
while the evolution theory entirely altered our conception of 
the inter-relationship of plants and animals, threw new light on 
the problems of adaptation of structure to function, and pro- 
vided us with a clue to the proper understanding of hitherto 
puzzling facts both in adult morphology and in development. 
Indeed, it is no exaggeration to say that during the middle 
decades of the nineteenth century modern biology was born. 
We must now attempt to sketch, although necessarily in the 
briefest possible manner, a few of the more important dis- 
coveries that were made during and after these critical years. 

The Life Histories of the Lower Plants. We have seen 
how, in the early part of the century, botanists confined them- 
selves very largely to the study of the higher plants, and how the 
various classifications that were put forward almost entirely 
ignored the lowest forms of vegetable life. The most that was 
accomplished was to divide them into three main groups, 
algae, fungi and lichens, while between these and the flowering 
plants came a heterogeneous collection embracing mosses, 
ferns, horse-tails and their allies, whose life histories were either 
quite unknown or very imperfectly understood. By the middle 
of the century, however, the life-stories of many of these lower 


forms had been traced out by various investigators, and, more 
especially, their sexuality had been established; although the 
connection between that phase in the humbler types of plant 
life and the corresponding process in flowering plants still 
awaited elucidation. That did not come about until 1850, 
when the great German botanist, Hofmeister, published his 
classic researches on " The Comparative Anatomy of the Higher 

Symbiosis and Lichens. Among the lowest plants, the 
lichens attracted considerable attention about the middle of 
the century owing to a unique structural feature they pre- 
sented. While in the main composed of a dense felt-work of 
colourless threads, or hyphae, like those of a fungus, and bear- 
ing reproductive organs of the fungi type, they also contained 
green cells called "gonidia," meaning ''offspring/' and which 
were believed to be special kinds of propagative organs (Fig. 115) . 
After a careful study of these bodies, the French botanist, 
Bornet, was able to show that they could be cultivated apart 
from the lichen, and were identical in type with the lower 
unicellular algaa. In 1868 Schwendener, professor in Berlin, 
asserted that the " gonidia " were really algae on which certain 
fungi had become parasitic. Subsequently Reinke, professor 
in Kiel, suggested that the 
association was not one of 
parasitism but of " symbio- 
^is," or "living together," 
with mutual benefit to each 
partner. Despite vehement 
opposition, the upholder of 

the dual nature of lichens won FlG - "S^'GOMBDXA" ENCLOSED BY 

the battle, and before the end 

of the century lichens disappeared as a separate class, and its 
members were distributed among fungi in accordance with the 
nature of the fruit bodies they produced, which were recognised 
as purely fungal in character. 

Similar cases of symbiosis were soon discovered, both in 
the plant and in the animal world. The greenish granules in 
the group of Protozoa, called Radiolaria, were also found to be 



symbiotic algae, as were also those in several other lower 
animals. Many forest-trees, heaths, orchids and other flowering 
plants were discovered to have fungi living in intimate union 
with their roots, the hyphae of the fungus acting in place of the 
normal root hairs, which were absent. This 
type of symbiosis was called " mycorhiza," or 

Bacteria. So long ago as 1540 the herb- 
alist, Valerius Cordus, noted the occurrence of 
tubercles on the roots of lupins, but made no 
suggestion as to their function. More than three 
centuries afterwards the same tubercles were 

FIG. 116. -ROOT found to be always present on the roots of the 
TUBERCLES OF , / n * i , i -r 

THE BEAN. S reat S rou P o:f flowering plants known as Legu- 

minosge, to which peas, beans, vetches, clover 
and such-like plants belong, and at first they were regarded as 
pathological growths (Fig. 116). About 1887, however, it was 
discovered that the swellings were occupied by colonies of 
unicellular fungi, i.e. bacteria, which invaded the roots from the 
soil, and caused a sort of inflammation or ''hypertrophy" in 
them. The bacteria (B. radicicola) were found to be able to fix 
the free nitrogen of the air, and thus accumulated in their 
bodies large quantities of that very essential element, which 
the host plant ultimately absorbed. This important discovery 
explained the value of the well-known agricultural custom of 
growing crops of lucerne and other leguminous plants on soil 
deficient in salts of nitric acid and ammonia, and " ploughing 
them in" instead of reaping them, thus providing a source of 
supply of nitrogen, for a succeeding crop incapable of forming 
such tubercles. 

Since, then, about 1893, and principally owing to the 
researches of Winogradsky, other nitrogen-fixing bacteria have 
been discovered (Azotolaoter and ClosMdiwn), which inhabit 
the soil and in the absence of ammonia or nitrates may do 
useful service in indirectly supplying plants with nitrogen 
from the air. It is important to remember that all plants must 
have nitrogen if they are to grow, but that in the great majority 
of cases this nitrogen comes from nitrates in the soil, not from 


nitrogen gas. Here again it was Winogradsky who showed 
how these nitrates are derived from ammonia, which itself 
results from the bacterial decomposition of nitrogenous excre- 
ment of animals or of dead plant remains. There are two 
distinct stages in the bacterial oxidation of ammonia, the 
first (i) brought about by a bacterium (nitrosomonas) giving 
nitrites (like KN0 2 ), which then (2) are oxidised to nitrates 
(like KN0 3 ) by another bacterium (nitrolacter). It is note- 
worthy that these two types of bacteria are " autotrophic," 
that is to say, as we have seen (p. 364), they build up their 
protoplasm entirely from inorganic ^materials viz. C0 2 and 
NH 3 so long as magnesium and other inorganic elements are 
present ; iron-bacteria (p. 364) appear to be similar, but other 
bacteria are parasitic, and can flourish only on suitable organic 
matter as foodstuff. 

Bacteria and Disease PASTEUR. This reference to these 
minute unicellular forms of plant life leads us to consider the 
enormous development that has taken place, since 1880, in the 
branch of biology known as bacteriology. We saw (p. 67) 
how the Dutch microscopist, Leeuwenhoek, as far back as 1683, 
discovered what he called " animalculse " in various fluids, 
putrescent and otherwise. In 1762 Plenciz, an Austrian 
physician, also studied the subject from the medical point of 
view, and went the length of saying that infectious diseases 
were caused by microbes floating in the air, and that each 
disease had a special microbe of its own. The whole subject 
was put on a scientific basis by the great French chemist and 
biologist, Pasteur, who from 1865 onwards showed that many 
of the phenomena, collectively called " fermentations/' were 
due to the activities of micro-organisms. In 1877 he studied 
the cause of the deadly disease of cattle called anthrax, 
and succeeded in preparing a vaccine or infusion from the 
anthrax bacterium which, when injected into healthy animals, 
while it gave them the disease in a very mild form, rendered 
them immune to the severer type. It is thus to Pasteur that 
we owe the foundation of preventive medicine by inoculation 
and the successful treatment of diphtheria, cholera, tuberculosis 
and a host of other more or less deadly ailments thus leading 


to a revolution in medical practice. Every surgeon in the 
middle of last century was only too well aware of the disastrous 
results that so frequently followed operations on the human 
body owing to the septic conditions set up in the wounds, but 
these untoward consequences were practically abolished by 
the introduction of the antiseptic precautions initiated by the 
famous surgeon, Lord Lister, who introduced carbolic acid 
(phenol) as a germicide. 

The study of bacteria was greatly facilitated by the vast 
improvements made in microscope lenses and methods of sub- 
stage illumination at the end of the nineteenth century, and 
also by the use of aniline dyes which enabled the microscopist 
to determine structural details to a degree not previously pos- 
sible. Culture methods of all sorts were also invented by Koch 
and others, and so it became possible to follow the life histories 
of various bacteria in the laboratory. The minuteness of these 
organisms was one of the chief difficulties that had to be con- 
tended with, and it was hard to realise that a speck of living 
matter, ^^ inch in diameter (a cell, too, without definite 
nucleus), could produce such calamitous results. 

Since Koch's time the study of bacteriology has opened up 
an immense field of knowledge concerning the nature of these 
microscopic plant cells which we call bacteria or bacilli. They 
are round-shaped (cocci), rod-shaped (bacilli), or spiral (spirilli), 
and their dimensions vary between as much as 60 /M and *2 /*, 
but the great majority are somewhere about i to 2 p. Their 
function is not, like that of green plants, to make their own 
foodstuffs by the exercise of the chlorophyll function (which 
is absent), but to live parasitically upon other food (vegetable 
or animal), by digesting which they thrive and multiply, 
in a similar way to that observed among the fungi (which also 
are not green). There are a few bacteria, like those producing 
nitrates (see p. 419), for example, which are able to build up 
their food from simple inorganic substances like carbon dioxide 
and ammonia, but the great majority are either truly parasitic, 
(using the living host as source of food) or saprophytic (living 
upon dead organic material) . 

The saprophytic bacteria are found in enormous variety, and 


they mostly discharge a very useful function in nature as 
scavengers, by destroying decaying and excremental matter of 
all kinds, whilst the other class (parasitic), also very numerous, 
are responsible for all sorts of diseases; though even here some 
of them, like Bacillus colim the lower intestine; seem to perform 
a useful and harmless function. The disease-bacteria, such as 
pneumococcus in pneumonia, owe their deadly effect largely to 
the fact that, while they are flourishing, they chemically trans- 
form the proteins of the blood or body into substances which 
are acutely poisonous (toxins) ; but anyone in a normal state of 
health is able to resist these dangerous organisms by pro- 
tective defences, since their white blood corpuscles (phagocyte 
or amoeba cells) are able to destroy bacteria. When bacteria of 
any kind become isolated from the moist food material (sub- 
strate) upon which they live, and dry up, they usually protect 
themselves from extinction by forming minute spores, which 
are resistant to drought. Moreover, in many cases these spores 
are not killed by heating to temperatures (say 100 C.) which 
destroy the bacteria themselves. Such invisible spores, of the 
common bacteria and moulds or fungi, are always present in 
the air, and whenever they fall on a suitable moist food-material 
they bud, develop, grow and multiply. And the rate at which 
all these microscopic plants multiply is prodigious. A bac- 
terium can divide into two every half-hour, and if that rate 
is maintained for twenty-four hours, a simple exercise in 
geometrical progression gives us the total number of progeny 
produced in that period as something like 70 million millions. 
Fortunately for us there are many conditions that prevent this 
appalling multiplication of small germs from being realised. 
A class of supposed bacteria, known as viruses, exist, so small 
(about '025 /t) that they can pass through the exceedingly 
fine pores of porous porcelain and they cannot be seen with 
the highest powers of the microscope, although their effects 
are only too well known, as with the virus that is believed to 
cause smallpox for example. 

We have already seen what an important part micro- 
organisms play in fermentation, indeed their activities have 
been exploited in many industrial concerns, such as wine and 


beer making, cheese manufacture, tobacco-curing and so on. 
In Nature also they play most important rdles. They are 
Nature's scavengers, for it is due to them that the waste 
products of animal and plant life, as also their dead bodies, are 
broken up and decomposed, thus not only rendering such waste 
innocuous to the living, but bringing important constituents to 
the soil for the benefit of future generations of plants. Year 
after year we are learning more about the structure, life history 
7 p RN and functions of bacteria, and 

.*"" 7 "". the importance of the whole 

Ernbryo \ ium subject has justified the founda- 

z $i te poi^ngium ^.^ ^ ^i^ O f bacteriology in 

Ovu A m S jjerm i our universities ; and the labours 

Sppre o; f hundreds of researchers has 
/ led to the production of an 

: >ROTHALLUS immense literature, extensive 

FIG. 117. LIFE HISTORY OF THE s^ugh indeed, to form a 
FERN. ,.- - ., -,, 

library of itself. 

Life Histories of Plants. About 1850 Wilhelm Hofmeister, 
professor of botany in Berlin, published a most important 
series of memoirs on the life histories of the higher cryptogams 
viz., ferns, horsetails, club-mosses and their allies. In these 
organisms he demonstrated the existence of two markedly dis- 
tinct phases in their life-cycles (Fig. 117) . A fern, for instance, 
produced myriads of spores, each of which gave rise to a 
small, flat, green expansion or prothallus, usually not more 
than |-inch in diameter. On this structure arose two organs, 
one of which the archegonium contained an egg-cell or 
ovum, while the other the antheridium contained numerous 
very minute mobile fertilising cells or sperms. The product 
of fusion of the ovum and sperm the " zygote " or yoked 
body grew in due course into a new fern plant producing 
spores once more. These two phases, he found, alternated 
regularly, and this "alternation of generations 1 ' was demon- 
strated as universal throughout the whole of the higher 
cryptogams, although varying in detail in the different groups. 

Hofmeister also traced the life histories of mosses and liver- 
worts, and showed that alternation of generations occurred 


amongst these forms also, but that in them the prothallus or 
sexual phase was the larger and more important, and that the 
asexual or spore stage was considerably reduced, and was, 
moreover, partially parasitic on the sexual. 

Not content with these discoveries, Hofmeister proceeded 
to investigate the life histories of the pine and its allies, and 
showed that in these forms the sexual phase became more and 
more reduced, until, at last, for the sake of protection, it re- 
mained enclosed within the spore wall. The 
zygote or young embryo (Fig. 118, E) was not 
only embedded in the prothallus, P, and spore 
wall, SW, but, further it was hidden within 
the sporangium, SP, the whole surrounded 
by an accessory integument or testa, T, form- 
ing the "seed," the characteristic reproduc- 
tive organ in all the higher plants or Sperma- SEED 

Phylogeny. These wide generalisations, linking up as they 
did the lower and higher plants, enabled botanists to reali5e 
that the key to the true understanding of the Flowering Plant 
lay among the hitherto despised and neglected Cryptogams. 
Published as they were about the time when the " Origin of 
Species " opened men's eyes to a new way of looking at organic 
nature, Hofmeister's researches helped materially to show 
that the connection between plants was phylogenetic or 
genealogical, and that linear classifications did not express 
their true relationships at all. The dogma of the constancy of 
species, which had been such an incubus on the shoulders of the 
taxonomists of the past, and which had driven them to all sorts 
of subterfuges to reconcile self-evident facts with preconceived 
ideas, slowly disappeared, and the plant world was visualised as 
a great tree whose roots were buried among the unicellular 
forms of the remotest past, while the tips of the loftiest twigs 
were the living plants of the present day. 

Fossil Plants. It will be remembered how Brongniart 
and others, earlier in the nineteenth century, did much to 
elucidate the structure of fossil plants, and came to the general 
conclusion that the oldest, Primary or Palaeozoic, rocks, and 


more especially the Coal Measures (p. 99), were characterised 
by the preponderance of ferns and fern-like types, and that 
seed-bearing forms did not appear until quite late in geological 
history. But in 1895 Williamson and Scott investigated a 
supposed fern stem which had been discovered some years 
previously in the coalfields of Lancashire, and found that it 
showed pronounced secondary thickening, like a modem larch 
or pine, a feature not possessed by any living fern, and yet that 
it had leaves like those of a fern. Further, in 1903, some of 
these leaves were found to bear seeds, quite like those of a 
cycad, an ally of the pine. Seed-bearing plants were thus 
seen to be enormously older than had previously been supposed, 
and after similar fossils from the Primary rocks, hitherto 
regarded as ferns, had been investigated, it became apparent 
that most of the vegetation of that far past epoch in the world's 
history did not belong to the fern alliance at all, but to an 
entirely new class of plants to which the very appropriate name 
of " Pteridosperms/' or Seed-ferns, was given. 

Cell Structure and Heredity. We saw how, in 1838, Robert 
Brown discovered the nucleus in the cells of the orchid-leaf, 
and how, in the years that followed, its presence was found to 
be universal in all living cells (p. 203). After the birth of the 
Evolution Theory, in 1859, attention was more and more directed 
to the discovery of the methods by which the characters of the 
parents might be handed on to the offspring. It was obvious 
that the answer to the riddle lay in the germ cells (gametes), and 
as a germ cell deprived of its nucleus soon died, it was concluded 
that the nucleus must be the essential part, the one that primarily 
concerned heredity. The ovum had a relatively large nucleus, 
and the sperm, or what corresponded to it, was composed almost 
entirely of nuclear material. Investigators, armed with micro 
scopes of high magnifying power and skilled in the use of all the 
apparatus of the laboratory, examining growth-mechanism, 
discovered that every time any cell divided, its nucleus went 
through very remarkable changes, collectively called "mitosis," 
from the Greek word meaning " thread." The nucleus when at 
rest (Fig. 119, i) was seen to consist of a very delicate membrane, 
M ; enclosing a network, N, composed of a substauce which had 



3 4 


a great affinity for certain aniline dyes, and which, for that 
reason, received the name "chromatin," the colourable matter, 
to contrast it with the less 
easily stainable ground sub- 
stance in which it lay. When 
division took place the chro- 
matin broke up into sepa- 
rate bodies called " chro- 
mosomes/' or colour bodies, 
usually resembling short 
hairpins, and apparently of 
very complex structure. 
One remarkable fact soon 
came to light viz., that the 
number of chromosomes was 
constant for any particular 
plant or animal, for the 
phenomena of mitosis were found to be fundamentally identical 
in the two kingdoms. Thus the number of chromosomes in the 
4ily and in the salamander is twenty-four, and in the human 
being forty-eight. The chromosomes arrange themselves across 
the equator of the nucleus, which at the same time loses its 
membrane, and from this equatorial plane faint streaks of 
threads converge to either pole. The chromosomes next split 
lengthways (Fig, 119, 2 C), and presently each half retreats 
along the striae to either pole (3 C'), where the fragments are 
reconstructed into daughter nuclei (4 N). A new wall is then 
laid down across the equator of the spindle (4 CW) ; so two 
new cells are constituted which then grow. 

With the germ cells the number of the chromosomes, 
which we shall call 2%, is halved at some period in the life-cycle, 
before the formation of ovum and sperm. This halving is 
obviously obligatory, otherwise the number of the chromosomes 
would be doubled every time the fusion of an ovum and a sperm 
took place, leading to an impossible condition of affairs. Let us 
examine two examples of this phenomenon. 

Alternation of Generations and Nuclear Division. The 
nuclei of the cells of a typical fern at division contain 2% 


chromosomes, and this number is maintained up to the moment 
of the formation of the spores. The nucleus of the spore is 
found, however, to contain only % chromosomes, and all the 
cells of the prothallus to which the spore gives origin, including 
the ovum and the sperm, have the same half number. When 
ovum and sperm unite, the number of chromosomes in the 
nucleus of the resulting zygote is, of course, 2#, and that 
number is adhered to in the cells of the embryo, seedling and 
adult, until spores are formed once more. By a curious modi- 
fication of mitosis, called " meiosis " or reduction, the number of 
chromosomes is reduced to one-half in the parent cell of the 
spore. It will be apparent, therefore, that the very obvious alter- 
nation of generation in the fern life-cycle is accompanied by an 
equally regular though hidden alternation in nuclear conditions. 

A very common seaweed, found on all rocky shores, is the 
"bladder-wrack/ 1 or Fucus. In this plant's life-cycle there is 
no alternation of generations, for there is no asexual stage at 
all. The nuclei of the body cells contain 2# chromosomes, but 
those of the ovum and sperm contain only x. The reduction 
in this case is effected at the moment of the formation of the 
gametes, so that the zygote regains the standard number zx. 
The condition in Fucus is that common to the whole animal 
kingdom, in which there is no such alternation of generations, 
as we have recognised to occur in the vast majority of the 
members of the plant world. 

Gametes as Transmitters of Hereditary Characters. 
From what has been said it would appear that biologists are 
justified in regarding the nuclei of the gametes as the essential 
organs concerned in the transmission of hereditary characters; 
indeed, no other means of transmission exists, but how that 
transmission is actually effected we do not know. It is a riddle 
that may be solved in the future, along with the equally 
mysterious problem how a minute particle of granular proto- 
plasm has the power of nourishing itself, reproducing itself, and 
responding to stimuli from within and from without. 

Modern Views on Evolution. Although a fierce battle, 
lasting for many years, raged over Darwin's famous book, 
" The Origin of Species/' adherents to the theory it propounded 


increased in number, and, by the end of the century, the doctrine 
of " Evolution by Natural Selection " steadily gained ground, 
and made its influence felt in every department of human 
thought. All the same, even its strongest supporters, men 
like Huxley, Hooker, Lyell, had to admit that, while the general 
principle seemed unassailable, there were very many confusing 
problems awaiting solution. For instance, physicists of the 
standing of Lord Kelvin found themselves unable to concede 
anything like the number of years that, according to the 
biologist, would be necessary for the evolution of the million or 
more types of organism now existing on the earth's surface. 
It is true that the views of the physicists have changed very 
considerably during the past generation, and an earth capable 
of supporting life might, they now admit, have existed many 
millions of years ago; but as recently as 1876, Tait, professor 
of physics in Edinburgh University, and a co-worker with 
Kelvin, postulated only 15 millions of years for the age of 
the earth as a habitable globe even for the very simplest 
organisms. But when the span of the earth's life was extended 
to anything between 1,600 and 3,000 million years (p. 354), 
instead of the paltry 15 million that Tait would concede, the 
difficulty of time might be said to have been got over. But 
there were other difficulties. 

No one can say how life originated on the globe (see p. 364), 
but it is certain that the earliest recognisable forms (the pro- 
tista) were very primitive unicellular vegetable organisms, 
and it is equally certain that from these lowly beginnings, 
over a vast period of time covering something like 300 million 
years, higher forms were slowly evolved in two great divergent 
branches, the vegetable and animal kingdoms as we know 
them today. The actual cause of this wonderful evolution is 
still to some extent a controversial question, and though 
natural selection and sexual selection postulated by Darwin 
may be the prime instruments in the progressive change, it is 
now realised that this is not all the story. 

Whatever may be the ultimate explanation, the broad facts 
of evolution are clear enough; they are revealed unmistakably 
ia the unbroken and indelible record of fossiliferous rocks, 


throughout the long range of geological time (see table on p. 483) . 
It is in the oldest (pre-Cambrian) rocks that the beginnings of 
life must be sought, but here the fossil record is virtually 
blank, because simple soft-bodied unicellular organisms, the 
protista, like marine alga and protozoa, flagellata, etc., could 
leave practically no permanent imprint, especially since these 
rocks have been often contorted and changed (metamorphosed) 
by heat agency. By the time the Cambrian rocks are reached, 
life is in full swing, and already of a relatively high (multi- 
cellular) order invertebrates like molluscs, the worm family, 
and crustaceans (especially trilobites) are flourishing, but all 
aquatic and nothing higher in the scale of life. The record is 
more abundant in the succeeding Ordovician, while millions 
of years later, in the Silurian epoch, which lasted over 30 
million years, the first beginnings of vertebrate (back-boned) 
life appear in the form of primitive fishes (elasmobranchs), with 
typical gills but with unsymmetrical so-called heterocercal 
tails (a long and short lobe as in sharks of today). In the 
succeeding (Devonian) epoch some new fish-like creatures 
(dipneusts) appear which, by living in mud that occasionally 
dried, had acquired the habit of utilising their swim-bladders 
as means of oxygenating their blood by gaseous exchange (air), 
instead of their now useless gills. These creatures had invented, 
so to speak, primitive lungs, and they were destined to usher 
in a new race of air-breathing animals, by emergence from 
their original homes (water) to land. In the next epoch, the 
long Carboniferous period, not only was the land conquered, 
but also the air, for now amphibians (like the frog tribe 
still partly tied to their watery home) appear as well as insects, 
derived from lower worm-like forms by progressive evolution. 
Towards the end of the Carboniferous period, reptiles, derived 
from certain amphibian stocks, were beginning to appear. 

Moreover, in multitudinous other directions progressive 
development was occurring, yielding a rich variety of new 
forms of life adapted to their slowly changing environment; 
also by now plant life had become air-breathing, and reached 
the comparatively advanced or complex forms of huge fern- 
like trees and giant horse-tails (see p. 424). 


From the point of view of human and mammal ancestry, 
however, amphibians were the most important living forms 
of the long Carboniferous epoch. They had developed four 
legs with five-fingered toes, from certain cartilaginous bones of 
the fins of their fish ancestors, and this endowed these creatures 
with the mobility requisite for existence on land. These limbs 
and toes and their entire skeletal framework have been retained 
by many of their descendant races, including mammals, to 
which they gave rise, though lost by disuse in serpents or 
modified as in the case of birds. Their immediate descendants, 
the reptiles, which were very varied and abundant some 20 
million years later in the Permian epoch, had emancipated 
themselves entirely from the water, so far as breathing was 
concerned, by elaboration of proper lungs; and during the 
succeeding Triassic and Jurassic epochs they became the 
masters of the earth. Enormous monster reptiles, like dino- 
saurs, roaming over the land, made life very precarious for any 
other types of animal. Nevertheless, during the late Triassic 
period primitive mammals began to appear, small and tenta- 
tive in type, evolved from smaller reptilian stocks, and still 
endowed with certain reptilian qualities, such as hatching their 
young from eggs laid outside the body. Pouched mammals 
(marsupials) like small kangaroos followed, and about the end 
of the Jurassic period, when the great reptiles had become 
extinct, true mammals began to appear (placentals), descended 
from some primitive pro-marsupial stock. The young were 
now born after a definite period of gestation within the uterus 
of the mother, and were sustained in their early infancy by 
milk sucked from their mothers' mammae. 

But in the Cretaceous age, which succeeded the Jurassic, 
these mammals were still only small and very primitive, living 
principally upon insects. Yet from this primitive stock, 
by an elaborate process of branching, all the multifarious 
mammal forms known today have descended carnivora like 
lions, tigers, wolves, etc.; herbivora like cows, sheep, horses, 
etc. ; rodents, bats, monkeys, etc. One of the representatives 
of this early stock took to an arboreal life, apparently for 
reasons of safety, and this new departure led to tremendous 


consequences. For it led to a great development of their 
grasping five-fingered fore-limbs, and to a greater use of their 
eyes, instead of the sense of smell, which was the principal 
sense used by other pro-mammals and mammals in scenting 
food, danger, etc. The descendants of this arboreal stock 
became tarsoids and lemurs, and from the tarsoids monkey- 
forms (primates) gradually evolved. This took place ap- 
parently in the Eocene period, and in the succeeding Oligo- 
cene already two great branches of primates had diverged 
(i) the platyrrhine or true monkeys of the American continent, 
and (2) small catarrhine or anthropoid apes of the old world. 
As Darwin foreshadowed, it was from the latter stock (now 
comprising the gorilla, chimpanzee, orang, and gibbon) that 
the human stem eventually branched off in Miocene times, only 
about 2 million years ago. 

No one, of course, believes that this human stock has 
descended from existing anthropoid apds; the truth is rather 
that it and the existing apes have all branched off from some 
arboreal ancestral form, which was the common progenitor of the 
entire group, in the same kind of way that worm-like forms 
were the common progenitors of molluscs, crustaceans, and 
fishes in Cambrian or Silurian times. 

All the old forms in the geological record are now extinct 
as actual species, and the existing species of all plants and 
animals must be looked upon as modern representatives, more 
or less changed, of the older forms which once flourished and 
were no doubt well adapted to their environment ; so that in 
the gigantic tree of life, which has been growing for over 300 
million years, we only see now, as it were, the terminal twigs. 

But the main branches of this great tree, with most of its 
sub-branches, can be clearly discerned in the geological record, 
and the recognition of this marvellous evolutionary development 
has finally disposed of the old dogma of constancy of species. 

If confirmation of the facts of evolution were required, 
they are to be found on every hand in the study of comparative 
anatomy, by which the homology of bony or other parts coin- 
cides with the closeness of genetic relationship; it is to be 
found in the existence of tell-tale vestigial and rudimentary 


parts no longer of any use, because the organism has changed 
its mode of life, but betraying clearly its past (phylogenetic) 
history; and it is to be found in the mode of individual (onto- 
genetic) development of the young, for example, of mammals, 
from the fertilised ovum, where the embryo more or less faith- 
fully recapitulates the history of its ancestors by passing 
successively through a fish-like and amphibian-like develop- 
ment. No rational interpretation is possible except that of 
evolution to explain the multitudinous facts of biology; in 
the light of this simple explanation, all the lines of evidence 
converge to a single focus and living Nature becomes intelligible. 
Evolution does not mean that change is necessarily progres- 
sive; in point of fact, it has been for the most part progressive; 
from the more simple to the more complex, from the lower 
to the higher. But it is not necessarily so; some species have 
practically stood still for millions of years, others, by changing 
their mode of life, have lost the use of organs (eyes, for example) 
once useful. Man himself has largely lost his sense of smell, 
and has by no means the range of vision that some lower 
animals have. Many forms have definitely retrograded by 
adopting parasitic habits, like intestinal worms, and evolution, 
so far as we know, is not guided by any conscious agency of 
design; it operates by fitting an organism to its surroundings 
and enabling it to live and breed. And though it is universally 
recognised that natural selection (i.e., elimination of the unfit) 
has played an enormous part, it would seem that geographical 
changes leading to isolation and prevention of interbreeding 
must also have had a great deal to do with the inception of 
new species leading to fresh branches. Neither of these causes, 
however, could operate without something to work upon, 
some definite advantage in relation to the environment that 
could be selected by Nature. Variation or mutation, a tendency 
which individuals sometimes show to depart from the average, 
seems to supply the clue, because any change, however small, 
which gave such individuals an advantage in the struggle for 
existence would lead to their increase at the expense of normal 
individuals; and it is probable that Cosmic rays (p. 269) have 
played a part in initiating such variation. 


Darwin regarded infinitesimally small variations in the 
organism as slowly accumulating from generation to generation, 
handed on by heredity; but the problem was: Of what possible 
value could these extremely slight changes be to the planter 
animal in their initial stages? Some experiments carried 
out by Professor De Vries of Amsterdam in 1901, seemed to 
suggest that new varieties, termed by him " mutations/' might 
spring into existence at a bound, so to speak, and hence that 
the creation of new species might take place much more rapidly 
than was generally supposed or assumed by the Darwinian theory . 

A far more serious criticism was advanced in 1885 by 
Weismann, professor of zoology in Freiburg. He denied the 
inheritance by the offspring of any characters acquired by the 
parent during its lifetime. This was striking at the very root 
of the Darwinian theory, although Weismann proclaimed him- 
self to be a confirmed evolutionist. His thesis was that a 
certain part of the fertilised ovum, both in the plant and in the 
animal, is, so to speak, set aside from the very beginning as the 
starting-point for the germ cells of the new organism, and to this 
hypothetical substance he gave the name of " geimplasm," to 
distinguish it from " somatoplasm," which was concerned solely 
with the formation of the body or soma. If the gerrnplasm 
was alone responsible for inheritance, it was obvious that modi- 
fications of the somatoplasm could not be transmitted, and that 
any structural feature induced by use or disuse, or by nutritive 
or environmental influences generally, never affected the germ- 
plasm in such a way as to cause the offspring to exhibit the 
modification the parent had acquired. 

Although there were many converts to Weismann's doctrine, 
it was apparent that there was no evidence to show that the 
germ ceUs were actually stable and lived a life altogether apart 
from the rest of the body, shielded from the influences affecting 
the organism as a whole, while positive evidence of the inheritance 
of acquired characters was rapidly accumulating such as, for 
example, certain cases described by Kammerer in 1925. 

In 1900 a new name burst upon the biological world that is 
now familiar to every student of the science the name of Mendel. 
Gregor Johann Mendel was born in Moravia, and, in 1843, 


entered the Augustine seminary of Altbriinn, subsequently pro- 
ceeding to the University of Vienna, where he studied science, 
In 1853 he returned to Briinn as a teacher, and finally became 
abbot of the monastery, where he remained till his death in 
1884. During the long quiet years of his residence at Briinn 
he took a keen interest in the local scientific society and pub- 
lished several papers in its journal, dealing with the experiments 
he had carried out in the monastery garden on the hybridisation 
of peas and hawkweeds, an account of which he gave in 1865. 
Whether it was because biologists did not attach any particular 

T* D 


T TCD) T(D) D T T(D)TtD) D D- F3 

| D F4 


importance to the abbot's work, or because the paper was 
hidden away in an obscure journal, the fact remains that these 
now classic experiments, and the epoch-making conclusions to 
be drawn from them, lay unknown for thirty-five years until the 
paper was discovered in 1900, and immediately attracted the 
attention of the whole biological world. Let us see what 
Mendel discovered and what conclusions may be drawn from 
his observations. 

Mendel's Law of Inheritance. The ordinary garden pea in 
cultivation shows a number of distinct strains or breeds. Some 
forms are tall, some are dwarf, some red-flowered, some white, 
some have wrinkled seeds, others smooth, and so on, and each 
strain, if self-fertilised, " breeds true/' In one set of experi- 
ments Mendel artificially crossed a tall strain with a dwarf 
(Fig. 120, TxD), and obtained a progeny, FI, all of which were 
tall, no dwarfs and no intermediates appearing. Apparently 
in this first filial generation tallness overpowered dwarfness 
(indicated in the figure by placing D within brackets), and 



hence, Mendel said, tallness in this case was " dominant " and 
dwarfness subsidiary, or "recessive/' He then cultivated all 
the seeds resulting from the self-fertilisation of these tall 
hybrids, but instead of getting a second generation, F a , of 
tall plants, he got a mixture of tall and dwarf, without inter- 
mediates, the tall plants being three times as numerous as the 
dwarf. He next self-fertilised all the offspring of this second 
generation (F 2 ) and cultivated their seeds in turn, and found that 
the seeds of the dwarf plants, D, developed into dwarf plants and 
continued to breed true in subsequent generations. He found, 
however, that only one-third of the tall plants viz,, T con- 
tinued to breed true, and that the other two-thirds, T(D), 
behaved exactly in the same way as the tall plants of the first 
generation, (Fi) viz., producing three tall to one dwarf. 

Expressed numerically, in the second generation, out of 
every 100 plants, 75 were tall and 25 were dwarf, and out 
of the 75 tall, 25 bred true, so that the final result was 25 tall 
breeding true, 25 dwarf breeding true, and 50 tall breeding tall 
and dwarf in the same proportions as those of the first genera- 
tion. Mendel found that this law held good for every pair of 
characters that he selected, each pair of characters being entirely 
independent of every other pair. 

Mendel now proceeded to formulate a " theoretical inter- 
pretation of this scheme, which he realised must be in terms 
of germ cells. He conceived of the gametes as bearers of some- 
thing capable of giving rise to the characters of the plant, 
but he regarded any individual gamete as being able to 
carry one, and one only, of an alternative pair of charac- 
ters. A given gamete could carry tallness or dwarfness, but 
not both/' 

In an admirable sketch of " Mendelism " Professor Punnett 
of Cambridge summarises the effect of Mendel's discoveries. 
He points out that hereditable variation is based in the gamete 
and not in the individual. Somewhere in the course of the 
production of the gamete there is added or removed the factor 
to which the new variation owes its existence. It appears as 
a sudden step, and not by gradual and almost imperceptible 
augmentation^ Once formed its survival is determined by 


natural selection; if of value in the struggle for existence it will 
be preserved, if harmful it will be eliminated, but if neither 
useful nor harmful, there seems no reason why it should not 

On the old view no new character could be developed save 
by piling up minute variations; but many characters exist 
whose utility cannot be explained or accounted for. On the 
newer view this difficulty is got over, for, provided the new 
variation is not directly harmful, it may persist, and thus we 
do not require to seek for a utilitarian motive behind all the 
multitudinous characters of living organisms. The function of 
natural selection is thus selection, not creation; it does not 
produce the new variation, it only determines whether it shall 
or shall not persist. 

It is worth noting that Mendel's work was published only 
five years after the (< Origin of Species " had left the printer's 
hands, yet Mendel makes no reference to Darwin's work, 
and similarly, "it is remarkable that, as far as one knows, 
Darwin never in any way came across Mendel's work" 

Problems in Metabolism. The problems connected with 
nutrition and the nature of the vital processes, in plants and 
animals, received much attention during the latter half of the 
nineteenth century. The most important of these problems lay 
in the fields of constructive metabolism, problems that cannot 
be said to be solved even today, although progress has un- 
doubtedly been made towards their solution. Let us first 
briefly review the general situation. 

The animal and plant alike are capable of feeding them- 
selves, and maintaining life by absorbing and digesting materials 
from without, but these materials differ very markedly in the 
two cases. The animal depends on a supply of complex organic 
food substances, which may be grouped as proteins, carbo- 
hydrates and fats. These complex bodies represent stores of 
potential energy which become kinetic in the organism, by the 
oxidation of their constituents, due to the process of respiration. 
They form the fuel that, on combustion, keeps the engine going. 
The seat of these activities is the mysterious substance proto- 


plasm, which was identified as common to plants and animals 
about' the middle of the nineteenth century 'the physical 
basis of life," as Huxley called it. Plant protoplasm also 
has to be supplied with complex food materials of a similar 
nature, but in the great majority of plants, what is taken 
in, from the environment, is inorganic viz., water and 
soluble salts, derived from the soil, and carbon dioxide gas 
from the air, the former absorbed by the root and the gas by the 
leaf. It cannot be too strongly emphasised that these inorganic 
substances do not form the " food of plants "a phrase so 
frequently met with in elementary textbooks of botany. 
Since these substances are already fully oxidised, it is manifest 
that they cannot be used as fuel; thus they contain ^no avail- 
able stores of energy, and therefore are of no service to the 
plant protoplasm. The plant has thus a preliminary task to 
perform from which the animal is exempt viz., to manufacture 
proteins, carbohydrates and fats required, not only for its own 
protoplasm, but, in the long run, for that (as food) of the 
animal also. It should be noted that the three great groups 
of organic compounds comprised in the generic terms, proteins, 
carbohydrates and fats, are the essential basis of all living 
material. They are very complex in their chemical constitution, 
(i) Proteins (proteids) are complicated compounds of colloidal 
nature, built up chemically by the linking together of numerous 
units called amino-acids, of which the simplest is glycine or 
glycocoll amino-acetic acid, NH 2 -CH 2 "COOH. Proteins there- 
fore contain nitrogen as well as carbon, hydrogen, and oxygen, 
and some of them contain phosphorus and sulphur as well. 
(2} Carbohydrates are complex compounds of carbon, hydrogen 
and oxygen, in which the two latter elements are present in the 
ratio of two atoms of H to one of 0. The simplest carbo- 
hydrates are the sugars, like grape sugar (dextrose or glucose), 
C 6 H 12 6 , and cane sugar, Ci 2 H 22 Oii, but many natural carbo- 
hydrates are very complex in structure, like starch and cellulose. 
(3) Fats, like carbohydrates, contain only carbon, hydrogen and 
oxygen, but they are built up on an entirely different type of 
structure. They are all derivatives of the well-known compound 
of alcoholic type, glycerol, CH 2 (OH)* CH(OH)- CH 2 (OH) 


(glycerine), combined with long-chain fatty acids (palmitic acid, 
stearic acid, oleic acid, etc.), in such a way that water is 
eliminated by condensation (one molecule of a fat being formed 
from three molecules of fatty acids and one of glycerol, by 
eliminating three molecules of water). When fats are treated 
with water in an appropriate way they break down into these 
components (glycerol and fatty acids) by the process of fission, 
called hydrolysis as we have seen (p. 399). Soaps are the 
sodium salts of these fatty acids. 

In the plant the " autotrophic " synthesis of proteins, carbo- 
hydrates and fats is in some mysterious way associated with 
the green pigment chlorophyll, so characteristic of the plant 
world; for ordinary non-green plants (fungi, etc.) are dependent 
on organic food supplies, as animals are, and are equally incapable 

10 20 30 40 50 60 70 

90 100 





of creating them. The continuance of life on the globe obvi- 
ously, therefore, depends ultimately on the activities of the 
green plant. Biologists at the end of the eighteenth century 
regarded the vegetable kingdom as subordinate to the animal, 
while the truth lies in precisely the opposite direction without 
the green plant the animal could not exist. 

Photosynthesis. What, then, is this all-essential green 
pigment, and how does it operate ? It may be remembered 
that Grew (p. 66) was the first to isolate it from the leaf by the 
aid of olive oil, and that Priestley and Ingenhousz (p. 206) 
noted that, when exposed to sunlight, oxygen was given off 
from plants containing it. In 1819 two French chemists, 
Peletier and Caventou, called the pigment "chlorophyll' 1 
literally " leaf green " a name by which it is now universally 
known. Two questions then arose, first, what was the substance, 


and, second, what was its relation to sunlight ? for, without 
sunlight, the chlorophyll was apparently powerless. 

The first step of importance was taken by Sir David 
Brewster, who found that an alcoholic solution of chlorophyll 
gave a very pronounced absorption spectrum (Fig. 121), showing 
a dark band in the red region, and almost complete obliteration 
of all the violet and most of the blue, with less marked bands 
in the orange and green. In 1864 Sir Gabriel Stokes found 
that chlorophyll was a mixture of two green and two yellow 
pigments, a discovery which was confirmed and extended by 
Willstatter in 1913. The pigments are (i), oc-chlorophyll, (2) a 
green to yellow-green one, ^-chlorophyll, (3) an orange-red 
pigment, carotin, and (4) a yellow one, xanthophyll. One 
curious point emerged from Willstatter 's work viz., that mag- 
nesium was the only metal present, although iron had for long 
been regarded as a constituent of the pigment, because leaves 
could not be induced to become green unless a trace of an iron 
salt was present in the soil in which the plant was grown. 

The importance of Willstatter's work lay in the fact that 
it showed a close chemical relationship between chlorophyll 
(a and /3) and heematin the basis of haemoglobin in the red 
blood of animals, whose function is that of an oxygen carrier 
to animal cells. Although there is so much difference in 
appearance and function, both types of molecule are built up 
on a similar atomic framework. This framework is a system 
of rings, each containing four carbon atoms and one nitrogen 
atom, and so these bodies belong to the heterocyclic series 
(P- 394)- A 11 ^ though the atomic side-chain appendages to 
these rings may differ in the two cases, the skeletal atom-link- 
ing is similar, with this important difference, that the nitrogen 
atoms of the ring systems are linked together by an intermediary 
magnesium atom in the case of chlorophyll, but by an iron atom 
in the case of haemoglobin. It is to the presence of iron in the 
latter that its peculiar behaviour with oxgyen is due. Though 
as yet incompletely understood, we can say that in haemo- 
globin the iron atom enables a whole molecule of oxygen to 
be taken up without the usual oxidation, which, normally 
occurs when organic compounds take up oxygen. The oxygen 


is instead loosely linked on to the haemoglobin molecule, ready 
to be given up to " acceptor " substances in cells, such sub- 
stances being themselves oxidised, while the " oxy-haemoglobin " 
acts merely as carrier or oxidising agent (p. 395), and reverts 
to haemoglobin after parting with its loosely-added oxygen. 

This action is, of course, of enormous importance to all 
animals possessing blood. We owe a debt of gratitude to the 
worms, who were the first animals apparently in the evolutionary 
sequence to "invent" haemoglobin; for it is the easy but 
necessary oxidation throughout their tissues that enables 
higher animals to secure the requisite energy for their complex 
life-processes; and this could hardly have been done without 
a mobile carrier. When the blood passes through the lungs of 
a higher animal, the haemoglobin contained in its red corpuscles 
picks up molecular oxygen from the air breathed in, and so the 
resulting "oxy-haemoglobin" of the arterial blood circulating 
to all parts of the body effectively passes on this oxygen to the 
cells to do its important work, before returning as venous blood, 
deprived of its surplus oxygen, to the heart and lungs (p. 61). 

Chlorophyll in plants exercises a quite different function. 
It does not circulate, but remains in one place, say in the 
chloroplasts of the leaves, to convert C0 2 and water into 
carbohydrates through the agency of light that is to say, to 
transform solar energy into chemical energy. And when the 
matter is considered closely, it is seen that chlorophyll does 
this by a process of reduction (of COg) , whilst haemoglobin effects 
the easy release of such stored energy by oxidation (to C0 3 ). 

About the same time that Stokes was elucidating the nature 
of chlorophyll, Sachs, professor of botany in Wiirzburg, was 
carrying out research on the first organic product formed in 
the leaf as the result of the activities of chlorophyll in presence 
of sunlight, and made it out to be starch. He showed that 
starch disappeared from the leaf at night and reappeared by 
day; and, by growing plants under variously coloured solutions 
enclosed in double-walled bell-jars, he came to the conclusion 
that the production of this carbohydrate took place most 
vigorously when the leaves were exposed to the yellow-red rays 
of sunlight. It soon became apparent that starch, a very 


complex organic compound, could not 'be the first product of 
"photosynthesis," as the process came to be called; and 
later research Indicates that cane sugar is the first recognisable 
product. Now cane sugar, C 12 H a2 O n , can be synthetically 
formed from a mixture of glucose and fructose (fruit sugar), 
both isomers, C 6 H 12 6 , by eliminating a molecule of water 
(condensation}} whilst starch is produced by the plant from 
glucose alone by condensation. In 1870, von Baeyer, 
professor of organic chemistry in Munich and afterwards in 
Berlin, put forward a theory which has not yet been super- 
seded. It had been shown, ten years previously, that a sub- 
stance having some of the properties of sugar could be obtained 
from formaldehyde, CH 2 0, now well known in the preservative 
having the name of "formalin" viz., by treating it with an 
alkali. Working on this basis, Baeyer suggested that the leaf, 
when exposed to radiant energy, seized, as it were, upon a 
molecule of carbon dioxide from the air and " reduced " it 
i.e., removed oxygen in presence of a molecule of water, to 
formaldehyde, oxygen being thus formed as a by-product, 
as Priestley and Ingenhousz had observed a century before. 
This photo-chemical change is induced in some way by the 
agency of the chlorophyll, functioning as transformer of solar 
energy into the chemical energy requisite for the change. 
Formaldehyde is one of those substances which chemists call 
polymerisable, that is to say, one which has a great tendency to 
combine with itself, yielding complex compounds by poly- 
merisation, Thus six molecules of formaldehyde could form 
the sugar called glucose or "dextrose/ 1 This sugar when 
deprived of water by condensation could then become starch. 
The series of reactions may be represented approximately by 
the following equations: 

CO, + H 2 = CH 2 + 2 ist stage. 

(carbon dioxide) (water) (formaldehyde) (oxygen) (reduction) 

6CH 2 = C 6 H 12 6 2nd stage. 

(6 mols. formaldehyde) (i mol. glucose) (polymerisation) 

* = (C 6 H 10 5 ), + nH 2 3rd stage. 

(large number of (one large molecule (condensation) 

glucose molecules) starch or cellulose) 


The reason why the starch disappeared in the dark can be 
explained by saying that it was reconverted into sugar, and 
transferred to other parts of the plant to supply nutritive needs, 
the constructive process being then in abeyance. 

Baeyer's theory of photosynthesis, which has been amplified 
in recent years, is probably not far from the truth, but it affords 
only the merest glimpse of the mechanism, underlying the 
multifarious synthetic processes occurring in Nature's great 
laboratory. Starch and cellulose, the chief carbohydrates 
of the plant world, are still more or less inscrutable photo- 
synthetic mysteries. 

The closely related problem, as to which of the solar rays 
were most effective in photosynthesis, occupied the attention 
of many investigators towards the close of the century, 
more especially Professor Pfeffer of Leipzig and Professor 
Timiriazeff of Moscow. The former held that the most 
efficient region of the spectrum lay in the yellow, and that the 
maximum of photosynthesis coincided with the maximum of 
illumination; the latter asserted that the really efficient rays 
were those in the neighbourhood of Fraunhofer's lines B and C 
(Fig. 121). Timiriazeff 's view seemed to receive support from 
the researches of Engelmann, who, in 1884, hit upon a very 
ingenious method of determining the position of the constructive 
rays. It was well known that certain bacteria were exceed- 
ingly sensitive to minute traces of oxygen gas, and collect round 
any spot where that gas is being produced. Engelmann argued 
that if such bacteria were introduced into water in which a 
filamentous green alga lay, and if the filament were illuminated 
by the solar spectrum, the bacteria ought to congregate wherever 
oxygen was being given off i.e., wherever photosynthesis was 
taking place. On testing his method he found that the 
bacteria swarmed chiefly round the spectrum lines B and C, 
as Timiriazeff asserted, but also to a less extent in the blue 
region near Fraunhof er's line, F, indicating that these were the 
waves of radiant energy that were chiefly responsible for the 
elimination of oxygen, and therefore, presumably, for photo- 
synthesis. % 

More recent research work by Baly, Heilbron, and Barker 


has reopened the whole question, and these workers have trans- 
formed carbon dioxide into formaldehyde and sugar by means 
of light in presence of nickel carbonate; but the problem is not 
solved even yet. As Timiriazeff wittily expressed it in the 
Croonian Lecture in 1903, we must be content for a little while 
longer to contemplate green leaves locked up in glass bottles, 
like the philosophers in " Gulliver's Travels," who, with the aid 
of similar apparatus, taught their pupils how to extract sun- 
beams from green cucumbers ! 

The Importance of Carbohydrates. The most remarkable 
point in the whole photosynthetic story is that, although the 
element carbon constitutes about one-half of the dry weight of 
any plant, the whole of this element is obtained from the quite 
trifling quantity of carbon dioxide in the air viz., 3 to 4 parts 
in 10,000. 

The space we have given to this. subject is not excessive 
when we realise its transcendent importance to the human 
race. Were it possible to collect all the carbohydrates that 
form the most substantial part of our daily food, we should 
find that they amounted to about 15 ounces for each individual. 
Most of this comes from cereals, of which wheat is the chief. 
Chemical analysis of wheat shows it to consist of about 70 per 
cent, of starch and sugar, 12 per cent, of nitrogenous material, 
2 per cent, fats, 2 per cent, of minerals and 14 per cent, of 
water and indigestible substances. The value of food to us 
depends primarily on the energy it yields on oxidation (p. 448), 
and if we adopt the "calorie " as our unit of measurement 
(i.e., the amount of heat required to raise a kilogram of water 
from o C. to i C.), we find that i pound of wheat yields about 
1,600 calories. It has been calculated that an average man 
doing moderate physical labour requires about 3,000 calories 
per day, so that, were wheat his only source of nourishment, 
he must be supplied with at least 2 pounds of that cereal in 
the twenty-four hours, or 730 pounds per annum. Of course, 
"man does not live by bread alone"; he requires other sub- 
stances as well, in which wheat is deficient. 

The world's wheat crop for 1924-25 was between 140 and 
150 million tons, so that this harvest could support approxi- 


mately 450 million persons . When we add the combined amounts 
of other grain and root crops, there is no difficulty in seeing 
whence thecarbohydrates come, that form the daily bread of the 
world's population. And yet all this vast food supply is manu- 
factured by the green plant from carbon dioxide in the air 
and dilute soil-solution, provided there is sunlight, whence 
the plant derives its energy. We need not wonder that the 
ancient Egyptians deified the sun as Ammon-Ra " the giver 
of life/' Perhaps the day may not be far distant when chemists 
and biologists will discover how to manufacture carbohydrates 
without the green plant's assistance, but that day has not yet 

The attempted Synthesis of Proteins. If the molecular 
structure of such relatively simple materials as cellulose and 
starch be thus shrouded in mystery, what is to be said of the 
still more complex proteins, which form the fundamental basis 
of protoplasm itself ? The difficulties connected with this line 
of research did not deter men like Emil Fischer from tackling 
the subject. He was able to show that both animal and plant 
proteins, on submitting to the mode of fission, called hydrolysis 
(p. 397), could be split up into substances called amino-acids, 
water being taken up chemically in this (hydrolytic) change. 
An amino-acid (p. 436) contains the group of carbon, oxygen and 
hydrogen atoms called "carboxyl," COOH (p. 390), along with 
another group, of nitrogen and hydrogen atoms, NH a . If these 
two groups are linked by removing a molecule of water, we get 
CO-NH, which chemists call a " peptide linkage." The 
various amino-acids are constituted out of one or more NH 2 
groups united with carbon chains carrying carboxyl, and 
Fischer was able in his laboratory to link a large number 
together giving " polypeptides," bodies which are regarded as 
probable stages in the construction of the molecules of 
proteins. To follow the line of investigation any further 
would plunge us in depths of organic chemistry which would 
only bewilder the reader, 

Enzymes aad Fermentation. The three essential groups of 
substances involved in the growth and nutrition of both veget- 
able and animal protoplasm are, as we have seen, proteins, 


carbohydrates and fats, but to render these available for im- 
mediate assimilation by, or incorporation in, the protoplasmic 
complex, they must be chemically transformed so as to enable 
them to be fitted into their appropriate niches in the molecular 
architecture of the animal or plant body. This chemical 
transformation, in its mode, is utterly unlike the manifold 
processes of the chemical laboratory, but it (i.e., metabolism) 
is rather akin to fermentation. 

The manufacture of wine and other alcoholic liquors had 
for centuries been regarded as a chemical process called " fer- 
mentation," and, indeed, metabolism in both plants and 
animals was somewhat vaguely ascribed to fermentative 
action. Little was known of these transformations until the 
middle of the nineteenth century, when Pasteur gathered 
together all the available data on the subject, and classified 
the ferments in two groups, " organised " and " unorganised/' 
According to him, organised ferments were in all cases living 
organisms bacteria, yeasts and other microfungi and he held 
that fermentable processes were always due to their growth 
and multiplication; unorganised ferments, on the other hand, 
were colloidal organic compounds secreted by plant and animal 
cells, each such ferment adapted to a specific purpose, but 
capable of extraction from the living body and of operating 
apart from it. 

During the years 1830-35 several unorganised ferments 
had been isolated, such as ptyalin from the saliva, which had 
the power of transforming insoluble starch into soluble sugar., 
equally in a test-tube as in the animal, diastase, which per- 
formed a similar duty in the plant, and pepsin, which dissolved 
proteins, breaking them down into soluble peptones, and so on. 
Later it was discovered that colloidal platinum i.e., platinum 
in a very fine state of suspension in water had somewhat 
similar powers; and, finally, in 1896, Buchner succeeded in 
crushing yeast cells under a hydraulic press and obtaining 
from the product an extract-filtrate containing no yeast 
cells, but which was yet capable of fermenting glucose i.e., 
dextrose or grape sugar into alcohol as efficiently as the 
living yeast itself, It thus came to be held that Pasteur's 


classification was not strictly scientific, and that ferments 
were probably in all cases non-living, whilst the so-called 
" organised " ferments were minute organisms from which 
the real ferments, contained in their cell-sap, had merely not 
been isolated. 

The term now generally applied to all such unorganised fer- 
ments is " enzyme," a word derived from the Greek zyme 
meaning " leaven/' From this leaven (yeast) the unorganised 
ferment had been extracted by Buchner. Many enzymes are 
now known and they are given names, ending in the suffix -ase, 
while the substance which they decompose (ferment) is gener- 
ally known as the substrate. 

Enzymes have certain remarkable characteristics. They 
are specific rather than general catalysts. Thus cane-sugar is 
transformed into glucose and fructose by the specific enzyme, 
invertase, discovered by the chemist Liebig in 1870; indeed 
one of their most remarkable features is their specific 
behaviour in relation to substrates.' In order to make this more 
clear it is necessary to refer to a property exhibited by certain 
organic compounds, especially those of bio-chemical origin, 
known as " optical activity." The latter is the power which 
these compounds possess of twisting or rotating the plane of 
polarised light (p. 116), when a beam of such light is passed 
through them or their solutions. The instrument used for 
detecting and measuring this rotation is known as a Polarimeter. 
It has been found that such substances can each appear in two 
forms, of exactly opposite but equal rotation, that is to say one 
form of the compound rotates the plane to the right (dextro- 
rotatory) and the other an equal amount to the left (laevo- 
rotatory). For example, two such forms of lactic acid, 
CH 3 *CH(OH)'COOH, are known, one called ^-lactic acid and the 
other /-lactic acid. It was le Bel and van't Hoff who first for- 
mulated a theory explaining such optical isomerism. Ordinary 
isomerism, which is a well-known phenomenon among organic 
compounds, is the appearance of two or more compounds 
(different) having the same percentage composition and empiric 
formula (p. 390), but different structural or constitutional for- 
mulae (different mode of atom linking). The case, however, is 


otherwise with optical isomerism, where the pair of isomers 
actually have the same chemical and physical properties as well 
as the same structure, but differ optically. The key to the riddle 
was found in 1874, by le Bel and van't Hoff, to lie in molecular 
asymmetry due to the presence of one or more asymmetric 
carbon atoms i.e., atoms linked by all four valencies to four 
different atoms or groups. The tetrahedral arrangement of these 
four atoms or groups (see p. 389) might be in a left-handed 
or in a right-handed order, differing much in the same way as a 
right hand does from a left, or a right-handed spiral from a left. 
This new kind of isomerism, due to differences in the arrange- 
ment of the same atoms in space, came as a shock to chemists 
and was at first ridiculed by men like Kolbe. But it is now 
well recognised and comes within the domain of a special 
branch, known as stereo-chemistry or the chemistry of space. 
Substances which do not contain an asymmetric carbon atom, 
like water, alcohol, acetic acid, benzene, etc., do not appear 
as space-isomers (stereo-isomers) and do not affect the plane of 
polarised light they are optically inactive. On the other hand, 
the majority of organic compounds associated with life are 
asymmetric in their molecular build and optically active they 
may appear in two forms related to each other as a right hand 
is to its image in a mirror (left hand), though in many cases 
only one form is actually known. And one of the most salient 
features about enzymes is that, when they are able to ferment 
an optically active substance, they can, as a rule, only act upon 
the one form and not the image-isomer, in much the same way, 
to use a simile of Emil Fischer, as an asymmetric key will only 
fit its own (asymmetric) lock. Where, as is generally the case 
with bio-chemical compounds, like carbohydrates and proteins, 
there are several asymmetric carbon atoms in the molecules 
the case becomes exceedingly complex, but Emil Fischer has 
largely unravelled this complexity for carbohydrates and to a 
less degree for protein derivatives. 

Cane sugar is dextro-rotatory, but on hydrolysis (p. 397) it 
is decomposed, by the assimilation of a molecule of water, giving 
a mixture of two isomeric sugars, C 6 H 12 6 , one glucose, dextro- 
rotatory, and the other fructose, which is not a space-isomer 


and which is tevo-rotatory, but to a greater degree than glucose 
is dextro-rotatory. The mixture is therefore lasvo-rotatory, 
and since the sign of rotation has been changed or inverted, 
the change is often called " inversion." The inversion by inver- 
tase (p. 445) is represented by the equation: 

C 12 H 2 Ai + H 2 = C 6 H 12 6 + C 6 H ia O fl . 

It may also be brought about by boiling cane-sugar solution in 
the laboratory, with a trace of an acid. It is important to note 
that the acid does not combine chemically with the sugar, nor 
is it altered or destroyed in the process; the H-ions merely 
stimulate and accelerate the hydrolysis (